IMHO, sixteen years and twenty five years of life equal lust. They are both very young. They have much to learn. Perhaps the young man truly finds her to be lovely and interesting, but let's face it there is a great gap in their intelligent thinking. He has nine full years of experience over her. She is all about the excitement of an older man paying her attention. It is probably a big deal to her. He is older and in her view, wiser. For him to pay interest to her is a very heady deal in a young girls mind. This view is only derived from my own experience and does not speak for all.
If their relationship has not become sexual, then that is good. If it has, then it is up to the older man to be very considerate of the possible outcomes of this union. Condom comes to mind! I don't know what the legal age of consent is in the different states. Frankly, most people who fall in lust don't go to the books to find out if it is legal to have an intimate relationship.
The older one, we can only hope, has enough on the ball to use discretion and common sense and a condom if common sense fails.