We don’t need these characters to grow up in real time. They can mature and have countless adventures but the core concepts should stay the same. Cap Marvel/Shazam is about a childhood dream come true. Not just putting on a cape and pretending to fly in the backyard but actually saying a magic word and becoming like Superman. That why Billy should always be young. If he’s a 20 year old and can change into a 30 year old superhero it takes away a lot of the fun that the character represents.