Ageing?
They only have three old players, and it is rare for more than one of them to start a game.
I think Neville's performance against Chelsea may signal the end of him in big games.
Well it was noted that in the Chelsea game their average age was over 31, obviously VDS needs to be taken into context at 37, but still. This was old enough for the English press to pick up on, with their 'pedestrian' jibes normally reserved for Serie A sides

umpkin:
They do need to replace Neville and Scholes though, this is a fact. Giggs is hanging in there. The main problem is that the guys replacing them are either injured (Hargreaves) or not cutting in (Anderson, Nani, Foster, Tosic, Berbatov all with varying underachievement). Valencia has been a good buy, Rooney carries the team at times and they have 3/4 of a strong defence. Fletcher also plays above his technical level, especially in the big games. They need more quality in my opinion, especially in midfield, at right back and another striker.
Anyway a little more with a doomsday article:
http://www.timesonline.co.uk/tol/sport/football/premier_league/manchester_united/article7092580.ece