I don't know how much you actually know about tennis, but I'll help you out. Borg retired at 25-26, McEnroe reached his peak in 1984, and when power players like Lendl, Becker, Stich, Agassi, Sampras, Ivanisevic came along, his chances diminished. Also, tennis is about match ups. He played serve and volley, and once technology changed and power players came to the fore, he started declining, also 22-27/28 are really the prime years in most tennis players, so his time was slowly running out. Chang was a one slam wonder. He won his slam when he was only 17 (think gymnastics). He had no fear, he just played and ran everywhere and the clay courts suited his style of play. He too, was overpowered, but mostly from the baseline. You don't really expect a a 5'7 guy to dominate in the 90's when the courts were much faster, do you? Courier was a grinder, pure and simple. He, one could say, overachieved. He won 4 slams, and I think he over strained himself and burned himself out, he basically peaked at 23 and has overshadowed by Agassi and Sampras. Hewitt was the same way. He relied heavily on his movement, his grit and trying to grind people down. His style of play caused all the injuries he has had since 2006. His time as a GS contender was done after that. His last SF at a major was when he was 24. Injuries heavily influenced the last 8 years of his career. I think he could have contended more without those issues.
Now to Cilic. He was ranked #2 in junior in 2005. He beat Tim Henman at 18, beat the defending Australian Open finalist, Fernando Gonzalez in 2008 at 19, and continued playing well, almost making the 2010 Australian Open final in 2010. He's won titles and made solid runs at several majors. He has always been considered a star in the making, a talented player with a big game. Yes, 25-26 is pushing the age barrier to win your first slam, in a sport that's built for younger athletes, but Cilic just might be in his prime now. Yes, he was an outsider, but it's not like he popped out of nowhere. He's beaten great players along the way in this tournament. Had he been a journeyman ranked outside the top 50, with no significant results to speak of, grinding people down, one would wonder. In terms of talent, he's always had the capability to win big tournaments, it's just now that things opened up for him. Plenty of players can get hot for two weeks and make a run at a major, but it's ultimately his renewed mental toughness that won him the tournament. I also think that in the modern game, where courts are slower, and there are more physical rallies, it's more hard to win when you are 19.