What activities can you start age 13 without already being behind?

[accomplishment]

I have twin DC and for various reasons they haven’t done much extra curricular stuff outside of school, apart from learning to swim but just basics no races or anything.
they don’t do any sport outside of school any more really.
just wondering whether I have left things too late for them to join in with stuff that they can make friends. I

Boxing

Don't bother with stupid martial arts classes that have belt progressions.
Get them into a proper boxing gym. Many will have beginner classes or just open sessions. Get them to go along and just muck in.
I think 13 is exactly the right age to get them started with it.

At 13 my daughter said she wanted to start ballet lessons - she last did ballet at 2 years old so I thought it was a non-starter.
I found a group for teenagers who are new to dance and it’s been fantastic! Twice a week she goes in a grumpy teen (15 now) and skips out, she absolutely loves it and is working towards point shoes.
13 isn’t too late to try something new.

Rowing

I was going to say Scouts.
Table tennis is another good shout.

Karate
Tennis
Kayaking
Park run
Cricket
Drama
Choirknitting
Crochet
Photography
Genealogy
Lots of things a newbie can make fast progress in. I had a new foster child of almost 16 2 years ago. He started learning to play cricket as older teen already played. He's now moved up 3 teams in 2 years. It's helped to improve his self esteem 100 percent. Team sports are good for developing cooperation, making new friends and raising self esteem. Regular sports training builds up resilience too.

Martial arts. Ideal age to start and the good clubs are always friendly.

Kayaking. Also if you have a bouldering/climbing wall near you, look to see whether they run classes.

Oh wow. These are great. Thank you. Will look properly

Axel throwing. Most places won't take people under 12.

That's very late to start riding, at least to do any sort of competing.

OP says it’s to have fun and make friends.

Musical instrument?

My DC have started Air Cadets and fencing at 14 and are looking into climbing. They never did much extracurricular activities when younger other than basic swimming.

Rowing

An instrument like brass, where they often start a bit later (or at least you can start later) because of lung capacity, teeth, etc., so they are less likely to be behind. And lots of different types of music groups that they can join while they're learning. And choir can be joined at any age, and they can learn to read music while doing it, if they want.

Dance classes for older beginners.

Calligraphy. Origami. Chess clubs. Warhammer. All often started by teens.

But you are not supposed to start MARITAL arts until 16

So? They don't need to compete to enjoy horse riding.