Very likely they're taking bets behind your back as to which one can bed you first. If you are flirting for "fun" then let me snap that illusion right now, they're not considering it fun, they're considering it as an open door to a young girls bedroom. Ultimately it's up to you how you'll proceed, but don't be under the illusion that it's just harmless fun, a guy in his 30's and a girl in her teens have nearly nothing in common and their attention is from a sexual angle, not a friendly one.