In fairness, the whole concept of U3A is that it is for retired people.
You have retired a good 10 years younger than the state pension age, so if you look at things that have been set up by, and for retired people, it is logical you will be a good 10 years younger than most of the people that attend.
There's an element of thinking about what you planned to do when you decided to retire. There's nothing wrong with going to groups with people that are older than you of course, but if you have an issue with it you perhaps need to look for things that aren't designed for retired people.
All that said, oddly enough I was talking to someone last week who goes, and thoroughly enjoys the things she does - learning sign language, and a walking group seemed to be her favourites, and she volunteers by teaching a skill she has. But she was saying how she can go along to talks about all sorts of things ...history groups, astronomy, music appreciation (she would say she's not musical), art appreciation - and there are lots of art groups and craft groups to do practical learning.
But, as others have said, there are lots of ways you can volunteer without needing to be in a fixed place at a fixed time each week.
Gardening at your local hospice or similar
Become a Treasurer for a Scout Group or District
Litter Picking
Supporting whichever charity you wish as a fundraiser - even shaking a bucket at an event as an ad hoc thing but I have a friend that does probably 10 or 12 events a year, things like checking runners in at a half marathon or 10k
Walking dogs for the Cinnamon Trust
Being a guide at a small local museum (I understand NT doesn't like flexibility)
Volunteer as a befriender - older people though something like MHA or Age Concern, or supporting a person with VI through sense , or physical difficulties through PHAB, or Care Leavers
Refurbishing tools through TfSR or even Men Sheds
Cooking / prepping / washing up on a rota at a lunch club or warm welcome
Volunteering in the library
Toddler Groups (yes, often run by Churches, but not religious, and you certainly wouldn't have to attend services if you didn't want to, but you'd be welcomed onto the rota at most I've had anything to do with
Seriously if you want to volunteer, there are 1000s of ways to do it.
But what about 'Walking .... {insert sport of choice} ' ? I know a few people that do walking football, and one that does cricket.
Or there are plenty of community choirs where you don't have to be able to read music.