The essential trick is to age without growing up. In this usage, 'growing up' means 'to achieve some final and persistent state at which progress ends.' The point of the game isn't to grow up in the sense of achieving such a state; it's simply to grow in the sense of getting better / refining / expanding / ripening - i.e., maturing.
Once you've convinced yourself you've finally 'arrived' or you're finally 'there' or you may now 'stand pat', you've got nothing more to look forward to than sitting in the middle of the road awaiting entropy's onrushing headlights. You're welcome to be as self-satisfied as you wish while waiting, but smug roadkill is still roadkill.
I've always envisioned my own final phase (assuming I'd survived to see it ... ) as an ongoing culmination rather than a static decline. If anything, I've been looking forward to it for a half-century now.
As a teen, I closed one of my poetry pieces with a line I'm still proud of:
"The succor of age is bounty come due."
In my late twenties, my then-wife would smile knowingly, shake her head in tut-tut style, and tell me I was already a quite weird (and dirty) old man who just hadn't fully grown into the role yet.
In my thirties, I finally grasped, and adopted as gospel, the motto of the 65 Club at the University of California Law School:
"Youth and skill are no match for old age and treachery."
Now, in my sixties, I'm still outrunning and outgunning younger colleagues in my chosen field of endeavor. The mass of experience, prior lessons learned, etc., accumulated over decades makes this incredibly easier now than it was then.
The cherry on top is that they now call me 'sir' and hold the door for me.
The most important thing to aspire to isn't wealth, toys, fame, etc. It's simply 'slack' (lack of pressure; wiggle room; flexibility; free choice).
The 'slack' entailed in being a valued contributor on my own terms hanging out at home is, IMHO, the most significant bounty that came due.
Keep it coming! ...