I usually keep my keepers until a very old age, sometimes right up to 40, provided they are still putting in the performances. But also my second and third choice keepers are also world class younger keepers, who I have as successors but also to keep them out of the hands of other clubs. It is a constant battle to keep them satisfied with cup matches.
Actually, I am a bastard for this, signing expensive players I really, really dont need just to stop other clubs getting them. I frequently have more top class players than can fit in my 25 slots.