I think it's because 30s is where a lot of people first feel age with concessions. It's right when you first feel that you heal slower, have to watch your joints more, etc. The stark contrast from your 20s when you feel progressively more bullet proof each year.
Yup. I bloomed a little late and had my social explosion throughout my 30s. But every year the next day after a late night out drinking hurt more and more. Aches start piling up, weight is harder to keep off, it feels easier to hurt myself, and slower to heal.
Keeping your health together and your body moving is one of the most important things.
But the rest of it is just in our heads. We can learn new things, meet new people, start new romantic relationships. But we have to want to do that, and put ourselves in positions where there are opportunities for those things to happen, and then take advantage of those opportunities.
Or - that's been my experience atleast.