Except he didn't play in a pro system, never took the ball under snap, and played in a conference who is know for horrible defense and best qb ever is Andy Dalton.
He just isn't ready, 23 or not, he needs time to learn. He looks good because they aren't throwing allot at him and he is going against limited defenses. Remember Kizer looked decent in preseason.
Let the kid earn the spot when he is ready, not be thrown to the wolves early. Now that doesn't mean if he started from day 1 he wouldn't be successful eventually, just means bringing him along slowly gives him the best opportunity to be good and good quickly.
Let's go down the list...
"He didn't play in a pro system" - There have been 27 QBs picked in the first round this decade and by my count only 6 of them (Ponder, Luck, Manuel, Bridgewater, Winston and Rosen) played in what I would consider a true pro style offense.
"Never took the ball under center" - Not only is this extremely common in the college game (guys like Goff and Newton never took snaps among many others), but it's less of an issue than ever before. 58% of all NFL plays were in the shotgun last year. If you focus strictly on passing plays, that number spikes up to 79%. These numbers are only going to continue to rise. The NFL is a passing league and it's a shotgun league. Learning how to take snaps under center is not difficult at all. Mayfield has only been practicing it for about six months and I haven't heard of a single issue in camp, nor have I see a single issue in two preseason games.
"Played in a conference who is known for horrible defense and best QB ever is Andy Dalton" - There's just nothing goofier when evaluating prospects than conference bias. Johnny Manziel DESTROYED SEC defenses, including Alabama, en route to winning the Heisman and he was a total and complete flameout in the NFL. Carson Wentz was an FCS QB who didn't play a single NFL defender throughout his entire college career and he was on his way to winning NFL MVP this past season. You either have the talent and work ethic to succeed or you don't. The conference you played in is pretty much irrelevant.
"He just isn't ready, 23 or not, he needs time to learn." - Why is 23 year old, 5th year senior, 47 college games under his belt Baker Mayfield somehow less ready than say Marcus Mariota? Mariota was less experienced, played in a similarly gimmicky offense and started from day 1. If you're good enough, you'll be able to handle it.
"Bringing him along slowly gives him the best opportunity to be good and good quickly" - Where is the evidence that this is ever the case? Here's all the first round QBs since 2010.
11 started/played week 1: Bradford, Newton, Luck, Griffin III, Tannehill, Weeden, Manuel, Winston, Mariota, Wentz, Watson
11 sat/waited: Tebow, Locker, Gabbert, Ponder, Bortles, Manziel, Bridgewater, Goff, Lynch, Trubisky, Mahomes
Of the 11 that started right away, 8 of them are still starting QBs and the other 3 are 2nd stringers.
Of the 11 that sat, only 4 of them are currently starting. 4 of them are out of the league completely.
"Kizer looked decent in preseason" - Kizer really didn't look that good in the preseason. He hit a few impressive chunk plays, but overall he was well below average. He completed just 51% of his passes. He held onto the ball too long to a frustrating degree and ended up with just a 72.7 QB rating.
Mayfield is the most productive and one of the most experienced college QBs of all time. I ask again, if HE isn't ready to play from day 1, than what college QB really is?