"Superman/Shazam: First Thunder" isn't very good. It feels like Winick needed to justify his Superman/Captain Marvel/Eclipso arc in the Superman titles by releasing a prequel. But that's not its main failing.
Its main failing is a complete disregard not only for established continuity, but also for common sense.
Post-Crisis, Captain Marvel was introduced into the DCU during the "Legends" crossover, shortly after Wonder Woman made her debut, following the Crisis. At this point, Wally West was the fledgling Flash, Dick Grayson was grown up and had adopted the Nightwing mantle, and maybe five or six years have passed since them. If Billy started at 10, that'd put him square at the age of 16, which he claimed to be in JSA.
"First Thunder" ignores this, instead suggesting that Captain Marvel debuted during the first year of the modern age of superheroes, before Wonder Woman, the Flash, or Martian Manhunter (some have complained that they also suggest that Batman debuted before Superman, but the text clearly says "around the same time" for their respective debuts, which fits with Batman and Superman's established origins via "Year One" and "Man of Steel").
Now, I can understand wanting to ignore "Legends," but let's think about this a little more critically. It's generally accepted that about 10 years have passed since the modern age started (Batman and Superman's respective debuts). If Billy started in the first year, he'd necessarily be ten years older now. I recall him saying in "First Thunder" that he's either 10 or 13, which would put him in his 20s now.
Captain Marvel kinda loses the "childlike superhero" quality when he can vote and buy porno, doesn't he?
I know they need to keep Billy young, but they really should have put some thought into that before "First Thunder." Between his FT debut and the modern day, the following things have to happen:
*Batman takes a ward, Dick Grayson, and trains him to be Robin. Dick grows up and takes a new identity, becoming a recognized superhero in his own right.
*Batman takes a second ward, Jason Todd, who apparently dies at the hands of the Joker, but actually grows up to become the new Red Hood.
*Barbara Gordon takes on the Batgirl mantle, abandons it, and is subsequently shot and paralyzed. She then serves with Suicide Squad and the Justice League as Oracle before forming the Birds of Prey.
*Will Payton becomes Starman, as does David Knight. Knight works with him briefly before Payton's apparent death, and Knight subsequently dies, leading to his brother Jack's acceptance of the role. Jack acts as Starman for over a year before passing the torch to young Courtney Whitmore, who becomes Stargirl. Stargirl eventually ends up in an aborted relationship with teenager Billy Batson.
*Batman takes on a third ward, Tim Drake, who becomes a respected young hero and eventually quits, paving the way for a fourth Robin to make her name and die, leaving him the role once more.
*The Justice League is formed, as in JLA: Year One. Some time after, Barry Allen dies in the Crisis, leaving the mantle to his sidekick, Wally West. Hal Jordan is stripped of his role, which is given to John Stewart and Guy Gardner at times, before they all resume as Green Lanterns and Hal goes crazy and kills a bunch of people, then dies himself, then comes back as the Spectre, then comes back again as Green Lantern. Also, Kyle Rayner happens.
*Superman encounters a Supergirl from a pocket universe, executes three Kryptonian criminals, and the Kents basically give the Supergirl therapy. She makes a name for herself as a superhero and lover of Lex Luthor II (a clone of the original, posing as his own son, since Lex had cancer and faked his death). Superman gets engaged to Lois Lane, then dies at the hands of Doomsday, to be replaced by four alternate Supermen. After the original returns, Steel and Superboy make names for themselves as solo superheroes. Meanwhile, Supergirl melds with the essence of Linda Danvers and discovers that she's an Earth-Born angel. After an adventure with the pre-Crisis Supergirl, Linda leaves the identity behind and goes to Bete Noire to eventually be forgotten because DC dropped that title and now Peter David has to invent a different backstory. Also, Young Justice. After that, another Supergirl appears and stays around way too long before vanishing into the well of horrible ideas from whence she came, and a third Supergirl shows up with some sort of bad Loebian mystery surrounding her origins. Power Girl fits into all that someplace, and we have to leave time for Electro-Supes, Y2K, and the marriage.
*A bunch of things happen in the pre-Crisis that are actually considered to be post-Crisis continuity (omitting stuff like alter-Earths and characters who shouldn't have existed), but only pop up when writers find it convenient to reference them.
I can suspend my disbelief and say "okay, that's a good decade of stuff there. A busy decade, but that could all fit into a decade." I can't suspend it enough to say "okay, Batman's gone through four sidekicks. Two have become adults. But Billy Batson's still a kid." I thought one of the consequences of Infinite Crisis was greater attention to continuity. Apparently, it doesn't apply to continuity that you can figure out just by using basic sense.
If you want to keep Billy Batson young, then you can't have his superhero debut back during the time of the big three, unless you say he was 6 years old when he met Shazam.