Everything you do is your choice, and it really doesn’t matter if science says your brain should have stopped growing by now. That’s just a generalization.
Indeed.
Stopped growing does not mean the brain stops learning, it just means it is done developing.
And less plasticity does not mean no plasticity.
Sure it is much easier for a 2 year old to learn new languages, and a lot easier for a 20 year old to train your brain to do advanced math or martial arts.
But that doesn't really mean that much, mostly means it takes a bit longer time.
Also, getting older means some parts of learning gets easier, from experience and technique.
And the other side of that coin is, your brain isn't done developing until about 25, so even if it had more plasticity before, it didn't reach its full potential until now :)
It's stupid to use your age as an excuse to not put in the effort to learn.