To me 25 seems fine. The fact that there currently are three bona fide GC contenders who are also white jersey eligible (Schleck, Gesink, Kreuziger) doesn't say to me that the age limit is to high, rather that these three guys are so incredibly good at a relatively young age. Within the grand scheme of the Tour (the best race, field wise, of the year with almost all the best stage racers present) I think being only 25 and having only 3 years or so of cycling with the big boys still puts riders at a serious disadvantage.
That Kreuziger and to a greater extent Schleck have been high up in the GC for the last three Tours now (and Gesink this year) is fantastic and only good for the sport. Schleck will, without fuck-ups, win his third white this year, perhaps the yellow along with it, like Contador did in 2007. Next year the white jersey will be, likely, a two way battle between Kreuziger and Gesink, both of whom will be even more serious podium candidates than they are today with 11 stages left to go. I think the fact that behind those great young guns there is no clear successor goes to show that it is a serious challenge to be high up in the TdF at 25 (or younger).
Who would be the early favorite for the 2012 white jersey? Robert Kiserlovski, Bauke Mollema, Peter Sagan.... Cyril Gautier maybe? Arbitrarily changing the age limit because there has been a wealth of great U25s at the Tour the last 4 years is stupid. Give these guys the credit they deserve despite their age, and move along IMO.