Most top teams always go for goalkeepers that's experienced age is but a number in that wing that's why many goalkeepers last in football till they're close to 40 the only thing that would guarantee a youth team goalkeeper some playing time is if he develops beyond imagination then he'll be graduated into the senior team as a second or third choice. Some team even loan them out to inferior leagues to get more experience and develope into the quality they need before giving them the opportunity as an alternative to the first choice.
Not many footballers are able to stay at their best level at that age and that is not just talking about goalkeepers but almost all.
However for players in the goalkeeper position it is much easier to adapt at a slightly older age because they do not need speed but instinct.
That is why there are some goalkeepers who manage to perform consistently until that age, but teams also try to get a much younger goalkeeper for the long term.
Young goalkeepers are not experienced in playing tough games so they need experience to reach the best level and that is why they are loaned out to other teams before they come into the first team.