Can they truthfully say they're happier now than in their 20s?
Hell yes.
My twenties I spent qualifying in my profession, building my career, buying a home, not earning as much, yes travel and nights out and freedom I didn't appreciate compared to being tied down with dc. I had friends but it was all quite superficial compared to the same friendships now (the ones that lasted) and the many friends I ditched along the way.
My thirties i got married, great, then those years were spent in a haze of sleepless nights, grumpy with my dh, exhausted, dealing with young dc, still building my career and trying to be taken seriously, working hard. Some good holidays though and my social life has been similar in my 30s and 40s because of dc.
40s so far shaping up well. Still in same home. Paid off mortgage mid 30s, career finally shaping up, the top job is in sight. Get taken seriously, still got work stress but deal better. Dc older, hard work running them to stuff but easier than when they were little.
Still feel young, definitely less physically fit as the kids have broken me!
I wouldn't want to be in my 20s or 30s now. I look at work colleagues struggling on the bottom of the career ladder and feel exhausted at how long it's going to take them to get where I am. I look at other colleagues just starting their families and am glad I am no longer dealing with that.
We are all on the same journey. It's not a competition.
I feel happy for a 30 year old colleague just getting married: pretty, stylish, glamorous honeymoon planned. So what? I've been there but I'm not going back even if I wanted to. Shes still got 14 years to grind through to get to where i am.
I look at the 23-30yo women on my team again just no way would I swap. Its rubbish being them but I don't begrudge them their fun. They don't want to be me (yet) and I don't want to be them (again).