[paw broon]

Oh yes, there were other examples of kids turning into adult superheroes. See my article in Sequential.
We have a couple on site. Thunderbolt Jaxon and Superhombre.

Link to the book: Captain Courageous Comics 6

[The Australian Panther]

Jeff Lemire's BlackHammer series turns a lot of tropes on their heads.
One of those is his character "Golden Gail'

Like Shazam, Golden Gail was blessed with incredible superpowers as a child. As she continued to age, her powered other self did not, in a weird twist on the Billy Batson/Shazam relationship. Gail is in her 50's now, but in the strange temporal prison she shares with her peers, Gail is trapped in the body of her 9-year-old self. Out of everyone in this list, Gail might have it the worst. Not only is she locked out of her old life and old world. She?s locked out of her old(er) body, too.

https://www.cbr.com/black-hammer-jeff-lemire-comic/

The problem is, she finds herself in a set of circumstances where she must act like she is a 9 year-old girl, even though she is actually a middle-aged woman. She doesn't accept it well.
I followed this series up to the first big reveal. That disappointed me, so I haven't paid it much attention since. Doesn't make my judgment valid tho.
Try it, you might like it.

If you like your Golden Age references straight, there is always Big Bang comics and their CC Beck and Mary Marvel homage character, Thunder Girl.
https://mollywilsonthundergirl.com/who-is-thunder-girl/ 

I'm a huge fan of Big Bang comics! Still out there and going strong!

Cheers!