We do a few smaller ones every year and I’m a decade older than you with chronic health issues. I wouldn’t/couldn’t do Glastonbury or any of the bigger mainstream festivals. Biggest one we do has a capacity of 10,000 including staff/crew, smallest is just a couple of hundred people.
Sticking to the small ones makes such a difference. The whole site is smaller, so not as much walking and if you pick the right ones they tend to be less commercial too. (Have a look on E-festivals to find some that suit your music taste.)
The gamechanger for us, once we hit a certain age, was getting a live-in vehicle. No lugging kit, we have our own loo, cooker, fridge, decent bed and with an awning, some shade and somewhere to shelter from the rain. Last couple of years having a fridge that runs on gas and has a little freezer section has also been great, as we had ice-cubes and ice pops on tap.
An alternative for some people is booking one of the glamping packages, as you get a real bed and mattress with them, but they are ridiculously expensive and you still don’t get a fridge or your own toilet.
I know of two couples that stay off-site in a hotel and travel in daily, but that seems like a bit of a faff to me. Although there’s Rebellion, where it’s all at an indoor venue, so everyone stays off-site anyway.
We don’t tend to bother buying much food from the catering vans, it’s always over-priced and poor quality. Sometimes we’ll buy something on the last night, when our stocks are running low, but we don’t eat from catering every day we’re there.
Dd has been going with us since she was 1 (now almost 15) and absolutely loves it.
Long gone are the days when we’d pitch up with a two man tent, a double sleeping bag and the clothes on our backs.