You have to take into consideration the source.
People magazine's readership falls 70% women and the average age of these women is 38. So the 30 - 40ish age range for sexy men seems appropriate for it's People's demographic. (Slightly cougarish.)Alternately, I found a list of Sexiest Men over 50...(actor heavy, but that's to be expected from IMDB)
Please explain to me, in what universe do we live in where Leonard Nimoy is considered sexy?