Pretty much: she is immature enough that she is still susceptible to grooming. Her brain isn't even fully developed yet.

By the time I was in my early twenties, I realized I no longer had anything in common with fresh-out-of-high-school girls. I lost interest in dating and sleeping with them, regardless of whether I found them attractive. In my thirties, I don't even enjoy hanging out with most people in their early 20s. We have nothing in common. They aren't relatable. The "grooming" here is taking advantage of her desire for acceptance and having her play the role he wants her to fill: a role closer to his wants and needs, not hers. He is the one doing the grooming, because he is the one with more maturity and power in the relationship.

Imagine they break up. Fifteen years pass, and she's now 34. Her friend says, "Remember that time you were 20 and slept with a 50 year old?" Do you think she she will look back fondly on that memory?