A 20 year old and 45 year old can have very different maturity levels. The 20 yo is barely out of their teens. That said, some 20 year olds can be very mature for their age.

What about socially? Is the 45 yo going to want to socialize with others in their late teens or early twenties which would be the social circle of the average 20 yo. Would they even be welcome? Or looked at as a creep? They may still be socializing with those they graduated school with.

Will the 20 yo want to socialize with other middle aged people, which could be the ages of the 45 year old's social circle. It would be like socializing with your Dad and his friends. Fun to do once in a while, but generally having fun with kids your own age is more fun.

It could end up being a very socially isolating relationship due to society judgement.

And having others assume you are the kid's Dad could sting!