Billy Batson is a kid who transforms into a superpowered adult when he says "Shazam". But when he grows up, does saying "Shazam" just give him the suit and powers or does it make him look any different?
Kingdom Come (1996) had an adult version of Billy Batson, who upon saying "Shazam" changed into Shazam/Marvel. I see no particular difference in his Shazam look than in the one he typically changes into when Billy's depicted as a kid.
This is very much in line with what "the wizard" said in Shazam! #1 (2013): he didn't make Billy older, he transformed him into his full potential. I guess that if you're going to throw punches at demigods (or even gods), "full potential" will always come in the form of a burly 30-something man1 regardless of the host's original body.
1 For a male host, at least. Mary Marvel, the female version of Shazam, doesn't look so burly.
Even in the Ordway interpretation of Captain Marvel, his superhero form was manifested from Billy's ideal self image, which was that of his own Father. So it can be assumed that Billy would grow up to be the spitting image of his Father.