The "this" was "can be a big distraction if you are trying to create something". Or, put more clearly, 12 year old boys need to understand that (1) it is getting time for them to start to take the rest of their life seriously, (2) for that seriousness, it is time for them to get into some hard work learning and doing, (3) being 'creative' eventually is important, and it is not too soon to get started, (4) for this seriousness, getting much involved with females can be a distraction from a little bit with little harm and maybe some benefit up to too much and a lot of harm.