Judge it on the two individuals, not their ages.
Nonsense. The ages are relevant.
It's not about "maturity". A 16 year old might behave maturely, and might share similar interests to a 20 year old, but there is a massive gulf in terms of life experience, self-understanding, and perspective. Even if they behave in a "mature" fashion a 16-year-old is still developing mentally.
There is a huge imbalance of power in such a relationship, which is why men that age are interested in teenage girls. They are so much easier to control and dominate than a peer, and much less likely to even realise that's what is happening.
Just look at how many teenage girls who are groomed, exploited and trafficked genuinely believe they were in a relationship with their abuser. The same happens with abused children who have been groomed. They believe they have a special relationship and connection with their abuser. They will defend them passionately and deny there was anything wrong.
It's only when they are much older and have more life experience that they begin to understand how wrong it was, even if it's only initially developing a gut sense of "now I'm that age I wouldn't even dream of doing what they did with me".
And as for him seeming nice, so what? Grooming extends to parents. People with vile intentions don't announce it, because they want to be accepted and to get away with it.