Like a lot of us on these forums I earn my living flying and also fly as a hobby. I usually fly between 300-550hours per year for work, another 100-150 for fun and am reasonably current.
I have, however noticed a marked difference in instrument scan and memory ability between the ages of 40 and 50. That may be because between age 30 and age 40 I was flying 800 hours per year for work and was therefore far more current, it may just be down to age, but certainly asking the question in the crew room a lot of pilots notice their skills slipping as middle age moves on, their knowledge will usually keep them from areas where they really need all their skills, but I don't believe anyone can be as good mentally at sixty as they were at thirty. I'm kind of relieved that I have to stop professional flying at sixty.
SND