It used to be that 50 was the age at which you were officially 'old', then it was changed to 60.
Now, I'm pleased to see that the W.H.O. (whose employees presumably have nothing better to do) have revised how we refer to our ages, and have issued the following (rather strange) new classifications.
18-65 Youth, or Young people
66-79 Middle aged
80-99 Elderly or Senior
100+ Long-lived Elderly
I've been feeling Middle aged for quite some while, and I'm pleased to see that my feelings were correct. For a while I'd worried that I was just plain 'decrepit'.