How Very Dare You, I retired by mistake (long story) followed by the death of my mother and a mini-stroke for me. I was severely depressed for over two years without realising it, until I found our local u3a and became its Treasurer.
Do not be put off by the few negative reports of unfriendly groups - the majority are just the opposite. Ours u3a is often compared to a neighbouring group as being welcoming.
There is no age limit to join a u3a - the key is that you should be in your "third age" of life having been a child and a working adult. We have lots of members who've moved to part-time working and join us to fill their spare time. It saved my life after working a 70-hour week and feeling lost.
Obviously you can join various "interest groups" (if there is space and it suits your level of ability) but if not, start your own group. Group Leaders aren't always the most knowledgeable of the group - a group of knitters can teach each other/share patterns/natter - the Leader simply arranges where and when to meet, keeps records etc.
Don't be put off by the word "University" either - some groups do involve learning of an academic nature, others are just platforms for sharing experiences. There are no exams and plenty of members have held manual jobs in their earlier lives. Basically, no-one cares what job you did or how old you are. I was 60 when I joined and very much the "baby" at the time. We have plenty of 80-year-olds who like a good laugh - and even swear at times! Some of them are way fitter than me.
If you feel, after a while, you would like to help on the Committee, I can assure you that it is very difficult to encourage people to join Committees, let alone take on a particular responsibility - of which there are lots, even if it's just been "refreshments co-ordinator" for the monthly meetings or coffee mornings.
Good luck in your quest - in a couple of years you'll look back and wonder why you were worried.
Meg