He finally admitted that he felt he was too young for kids yet. (One thing I truly respect is...he's always been VERY honest).
These statements don't reconcile. You claim he's honest, you had a discussion about children, he must've agreed, you have one, and now he's not sure? That doesn't sound like honesty to me. Or am I missing something?
As to the issue of will he "grow out of it"? My guess he'll grow out of it by having an affair or going off and doing exactly what he wants. He has an intersting view of those wedding vows.
Frankly, the fact that he's asking you about it, might even be a way for him to sound out how you're going to react when he tells you he already has...
He doesn't seem to see fidelity as a cornerstone of successful marriage. Does it have something to do with him being younger? Nope. Has something to with him having a lack of character... Read the material on this site about the damage infidelity does. And note the infidelity is considered one of the *most selfish* acts. Doesn't have anything to do with age or gender or anything.
It doesn't appear like you told him that you didn't find it acceptable and haven't gauged his response. Are you afraid of what's going to happen if you stand up for the institution of marriage?
And ultimately it isn't an issue of age at all, it's an issue of what's right. Do *you* believe it's OK for him to do this? Because if you do, then there's no real reason to continue. Do you believe that somehow your Marriage si going to be strengthened? That later on when your daughter is older and looks at you and says, why isn't daddy sleeping at home, and talks about all his other girlfriends, that you're going to be able to look her in the eye and tell her, "Honey, letting your H do whatever he wants is a *cornerstone* of a good marriage, and when you're grown up and married, you feel free to do the same, either by sleeping with other W's H's, or going out and banging anything that moves if you feel that urge"...
Get real.