Trouble with Goalkeepers, its so difficult to for them to gain experience. Top teams wont trust goalkeepers until they have decent experience.
Which is probably why we see late developers. You always hear pundits saying keepers arent ready until they are 25 or older, but this is because they don't get a chance due to keepers playing on close to their 40s when most other positions early 30s is when the downwards spiral begins.
Even us, we had 2 keepers in their mid to late 20s but we decided to get a close to aged 40 Reina in instead.
Young players at big clubs get hyped, so it makes it easier for the likes of Scott Carson and Sam Johnstone to become first choice at clubs like Villa, when in reality better keepers were already on the books in both cases.