If I was a serious somewhat mature child, does that mean I get to be an immature adult?

Just be true to who you are, unashamedly so, as long as you don't cause others harm. <3

Yes, and you get to do so without the baggage. What a win-win :)

A little nonsense now and then, Is relished by the wisest men.

I seemed to lighten up once I hit my 20s and even more so at 50...

Yes it is.

You be you. Rude is not a direction I would favor.

Rude... if the person/situation deserves rude, then go Pro Level Rude on them, but always speak your mind.

Plenty immature children who are still immature adults. Imo, you just get to see the strings.

Grow old disgracefully at your leisure i say.

No thats just rude and outspoken