Justin Bieber - LOVE BITES

Justin Bieber has just learned what most men realize at his age ... hickeys are for kids.

Truth be told, we don't know for sure it's a hickey, but let us deconstruct. JB was poolside Friday, and strategically covered up his neck with a towel.

He was there with Kendall Jenner and Kourtney's kids. Others may have been there as well.

When he went to Mastro's Friday night, he lost the towel and the marks were apparent.