Just because van der Sar (and Lehmann?) did it, doesn't mean every keeper is going to be good/great until there late 30s.
The fact that goalkeepers are good when they get older is also somewhat invalid imo. Bayern bought Neuer when he was 25, De Gea was great when he was 20 and was bought by Ferguson. Courtois and Oblak are not even 25 yet.