I'm 65, retired. I'm more physically prepared now than when I was younger and busy with work and kids. I have time and money now. I see plenty of other backpackers in their mid 60s, but hardly any backpackers in their 70s. I've been doing a lot of backpacking in recent years because I know my time is running out.

I just got back from 11 days hiking in the Grand Canyon. My left knee hurts, so maybe time is running out...

Anecdotally the biggest factor for that, barring any acute health issue, will be your activity level between now and then.