It's also your muscles, which won't be old, and the on-going feeling of a ticking biological clock driving people to say "kids, now, before it is too late".
It's also how much sleep you need, how much energy you have.
It's also your metabolism, your ability to recover from light injuries, to exercise, to excel at sport.
It's also neuro-plasticity.
We don't have good examples to even guess if world-views (or fashion choices) lock in place because of brain age, or because we've seen too much and become bored.
But personality, that we can already change with chemistry; it's not a reason by itself for anything.
> Looking at my children grow older day after day is the thing that bring me the most joy and sens of meaning in my life.
Good for you :)
But for me, I wish I'd had kids already, but I have many other joys until I find someone who can help me with that (most of my friends are deliberately child-free). If I became ageless, it wouldn't be a worry to wait.