Here's a complete list of riders in the last 50 years who had their first GT top 10 at the age of 30 or above.
Tour de France
Geraint Thomas (1st, 2018, age 32)
Daniel Navarro (8th, 2013, age 30 - his 30th birthday was three days before the end of the race)
Thomas Voeckler (4th, 2011, age 32)
Jean-Christophe Peraud (9th, 2011, age 34)
Chris Horner (9th, 2010, age 38)
Christian Vande Velde (4th, 2008, age 32)
Cyril Dessel (6th, 2006, age 31)
François Simon (6th, 2001, age 32)
Roland Meier (7th, 1998, age 30)
Alberto Elli (7th, 1994, age 30)
Giancarlo Perini (8th, 1992, age 32)
Raymond Martin (5th, 1980, age 31)
Giro d'Italia
Yuri Trofimov (10th, 2015, age 31)
Przemysław Niemiec (6th, 2013, age 33)
John Gadret (3rd*, 2011, age 32)
Michele Scarponi (4th, 2010, age 30)
Marco Pinotti (9th, 2010, age 34)
José Enrique Gutiérrez (2nd, 2006, age 31)
Victor Hugo Peña (9th, 2006, age 31)
Patxi Vila (10th, 2006, age 30)
Marzio Bruseghin (9th, 2005, age 30)
Tyler Hamilton (2nd, 2002, age 31)
Andrea Noè (4th, 2000, age 31)
Heinz Imboden (8th, 1995, age 33)
Massimo Podenzana (7th, 1994, age 32)
Claudio Corti (5th, 1986, age 31)
Üli Sutter (10th, 1978, age 31)
Ronald de Witte (9th, 1977, age 31)
Vuelta a España
Michael Woods (7th, 2017, age 30)
Laurens ten Dam (8th, 2012, age 31)
Daniel Moreno (9th, 2011, age 30 - like Navarro he turned 30 during the race, this time on the second rest day)
Xavier Tondó (6th, 2010, age 31)
Paolo Tiralongo (8th, 2009, age 32)
David Moncoutié (8th, 2008, age 33)
Egoí Martínez (9th, 2008, age 30)
Ezequiel Mosquera (5th, 2007, age 31)
Félix García Casas (8th, 2002, age 34)
David Plaza (6th, 2001, age 31)
Claus Michael Møller (8th, 2001, age 33)
Roberto Laiseka (6th, 2000, age 31)
Gianni Faresin (9th, 1997, age 32)
Tony Rominger (1st, 1992, age 31)
Jean-Claude Bagot (9th, 1989, age 31)
Eddy Schepers (9th, 1988, age 32)
Pierre-Raymonde Villemiane (6th, 1982, age 31)
Jørgen Marcussen (4th, 1981, age 30)
Ventura Díaz (9th, 1974, age 36)
Roger Swerts (9th, 1973, age 30)
As you can see, a lot of these placements are low end top 10s, mostly placements from 6th to 10th, suggesting these are riders who were either backup options for their team or were simply not strong GC leaders but were able to compensate with nous and experience to get into the top 10 without threatening the podium. Many of them are also one-and-done top 10s; of those who hit the top 10 for the first time after the age of 30 above, several never made it again, and only a small handful became regulars up there - Rominger, Mosquera, Moreno, Scarponi, Bruseghin and Peraud are perhaps the main ones to point to. It's also harder to achieve this feat at the Tour - the Tour accounts for 25% of the number up there; the Giro is a perfect 33,33%, so the Vuelta accounts for 41,67% Of the 48 names above, only 4 have actually won a GT - and Thomas is the only one for whom that GT is the Tour; Tony Rominger won three Vueltas and a Giro, Chris Horner won a Vuelta, and Michele Scarponi won a Giro through forfeit after Contador was removed from the results sheet. Horner is a massive outlier even among this list which exclusively deals with late bloomers.
There is also a significant increase in the number of these post-30 maiden top 10s in recent times - of the 48, over 75% are since 1990, almost 66% are since 2000 and a little under a third are since 2010. It is not surprising that in the pre-Wende days, the Vuelta is the most common place for experienced stagers to get their first GT top 10 (6 first timers as opposed to 3 at the Giro and 1 at the Tour) seeing as it was by far the least established of the 3, and it is really since the 1980s with the takeoff of the skiing industry and the reintegration of Spain into Europe that its GT status is affirmed; the numbers start to be more in line with the other two GTs from that point onward. I would say that the increase in importance of the World Tour and its predecessors plays a large role in that, in that placements are increasingly important and so experienced riders drifting backward slowly are now quite a useful commodity as the ability to "Zubeldia" a top 10 is quite an effective tool for a team's continued World Tour ambitions. Also, I would argue that the increased quality of domestiques means that riders are better protected for longer; we may decry the lack of long-range action and aggressive racing, but more energy saved in the bunch is probably helpful in increasing riders' longevity.
In the early 90s you had a bit of an exception when you have the riders from the former Eastern Bloc who weren't previously able to ride the Grand Tours (other than a small handful of Soviet riders who did the Vuelta in the late 80s) although even they were mostly late 20s at their first GT (Ugrumov 29, Jaskuła 28 for example); they follow the example of Gösta Pettersson, who is probably the best example to point to as a precedent for the kind of performances that Wiggins and Thomas have put in after converting to being climbers late on; he didn't turn pro until he was almost 29, and top 10ed both the Giro and Tour in his first pro season (thus excluding him from the above list), then won the Giro in his second; he did however mostly race road as an amateur, though he did join in the track along with his brothers at times, such as the 1968 Olympics; his road palmarès as an amateur included some major stage racing credentials, however, winning the Tour du Maroc and the Tour de Tunisie as well as the Milk Race when it was two weeks long, and coming 2nd in the mountainous Tour de Yugoslavie, so he's not a perfect comparison. He is perhaps more directly comparable in the modern day with somebody like Niemiec, who for personal reasons (a private family connection) signed a much longer than is normal contract with a third tier team and therefore didn't get to do a GT until he was 31, or Tondó, who took to the sport late, and was buried by bad timing beneath Puerto exiles who were limited to lower level teams, that blocked his route to the top until he was nearly 30; Mosquera belongs in the same category, not having the chance to race the Vuelta until 2007, but obviously he also pushed his luck too far trying to win the race in 2010 and fell foul of doping controls.
There's also a lot of other convicted or otherwise known dopers there too - Hamilton, Vande Velde, Meier, Gutiérrez, Hesjedal, Scarponi (who may be unique in having served his ban prior to his appearance on this list). Very few podiums among these first top 10s, though there is the occasional late convert to at least this side of the sport (Jice Peraud being the most recent example of this, he of course later did podium) as well as those who didn't turn pro until a late age back in the pro-am days; Michael Woods is of course another outlier, having been in a completely different sport. There are quite a few fluke performances and ones where large time gifts played a role (Voeckler, Martínez and Dessel, for example, and I can't remember if Pinotti was in 'that' break). Quite a few of the most notorious 'late bloomers' in world cycling, regardless of their reasons for that, actually had their maiden GT top 10s at 28-29 - Ugrumov, Wiggins, Hesjedal, Joaquím Rodríguez, Pettersson - so in some ways using 30 as the cut-off is slightly arbitrary as if it somehow makes, say, Moreno or Navarro, who both finished at the bottom end of the top 10 in a race they had their 30th birthdays in, more significantly late bloomers than Wiggins or Hesjedal (Moreno had finished 12th, 12th and 11th in his 3 Vuelta rides prior to being 9th in 2011, so he was actually very consistent and it's his 5th the following year that's more of an outlier, whereas Hesjedal's and Wiggins' best GT results prior to their successive maiden top 10s were 47th and 71st respectively, so you'd say Moreno's performance is much less inherently eye-opening once that context is added - though I believe Moreno was one of the riders connected to Jesús Losa back in around 2009-10), but I thought its position as a milestone birthday and a point at which historical provenance suggests riders ought to start to decline made it a useful starting point, though of course the point at which a rider actually does stop improving and start to decline is never going to be at a uniform point for all athletes.
The number of first timers making the podium, as Thomas did, is pretty small too - counting Gadret is debatable because of Contador's presence in 2011, but even with him, there's literally only Thomas, Rominger, "Búfalo" Gutiérrez, Hamilton and Gadret among those over 30 in the last half a century who have had their maiden GT top 10 be a podium - not exactly company Thomas will be clamouring to be conflated with I should imagine - and furthermore, interestingly, Thomas is the only rider to achieve that at the Tour de France. And not only that - but he's the only rider to never hit a GT top 10 until 30 years of age, and then have their maiden top 10 be a GC victory.
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