Salary isn't the only indicator of the strength of a player, but a 30 year old with better skills will always be better than 25 with lesser skills (assuming game shape is equal). There's no point dropping a 31 year old who is much better for a 25 year old simply because of the age. This isn't real life where you need to get young players through to develop - all they get here is a small spike in experience which, whilst probably useful (and mysterious), isn't enough to justify their inclusion.