Jill is in her 3rd year in college, she is now 21. So three years earlier the thing with Bella could have happened (and its kinda obvious that something happened between them, cause Jill herself said so in the bed with Bella) and she would have been 18 at least late 17...Honestly, If I were to guess I would say it was with Lana and not Jill. From how it seems Bella and James havent been together in quite sometime. If thats the case going by Jills age she could of possibly been under the age of 18 if they fooled around. Im gonna go out on a limb and say Jill who is super vanilla wouldnt be doing things at that age.
Lana was more of the rebel of the family and would have been closer to Bella in age. It would explain why Bella and Jill are so close now because of how good of friends Lana and Bella were before hand. Also could explain why Lana hasnt been around, She left after breaking up Bella and James marriage. And why Bella has stayed so close to Jill because she knows how much Jill loved Lana and is trying to be the sister figure since she left.
But nevermind, the underage thing doesnt matter anyway. Cause i dont think we will ever see Jill to have sex with any other than the MC... and just to mention it doesnt bother anyone.
Besides... we already saw "underage" sex if you want to be exact... between Zoey and MC for example, and i guess, Maya and Josy werent 18 when they first had sex in the tent.