At 19, some people have done a hell of a lot of growing up than others. Trust me on that.
I used to say and think shit like this when I was younger, and did really believe it, but I was wrong too.
It's not about an age gap in years being a problem, it's about the placement of it (i.e. teen and late twenties, rather than late twenties and late thirties) and the imbalance of power resulting from that.
Most people in imbalanced, controlling relationships don't realise until it becomes utterly destructive - and often even then by that point they've become so conditioned to accept it as normal that they still don't see it.
Op, if you're as competent at spotting early warning signs of abuse as you say then why are you not familiar with the observation so many here are making about the imbalance of power inherent in any relationship where a much older man has entered into a relationship with a woman barely a legal adult?
It's not a judgement on you for people to hold concerns about his behaviour and his choices or the risk it exposes you to.
Humans don't actually reach maturity until mid to late twenties. That is why age gap relationships positioned between the ages we are talking about here are potentially so very problematic and concerning.
Doesn't matter how many people pop up to say how grown up they considered themselves at 19 - plenty of ten year olds view themselves the same way, but it doesn't make it an objective assessment of their ability to engage with much older, more experienced people on equal terms.