It's not exclusive to older people (teenagers can do it too but it's not recommended)
It's called "having a baby".
My daughter weights 28 Ibs now and I have to lift her a couple dozen times a day. My upper body has definitely improved.
But seriously, I've found staying fit is MUCH harder than it was 5 years ago but still possible. The hardest part is finding the time. I don't know many 30-somethings (especially in tech) that can work out for a half hour uninterrupted, nevermind an hour.