13 was from interviews a couple of years ago because at the time he would have been 15. He said 2 years younger so now that he's 18 the minimum is 16. However he might of changed his mind, but still highly unlikely he'd date someone 13 right now. If it was a 5 year difference and he was like 25, then maybe.
I feel like im really late or something since how its 2018 and Luke is 21. Ikr, crazy. But now I feel like he would only date someone close to his age, like 19 AT THE MOST. Im like a gazillion years younger than him so im like *tears* but I don't think he is really in the mood for a relationship right now