There is nothing wrong with putting them together once she is out of heat but you'd have to remove her or him again 2 weeks later because she'd start coming on heat again, lasting for an average of 3 days. When she's coming onto heat and coming off, the boar will probably try to ride her and if she's not ready this can cause cuts/chaffing and stress to the maiden/gilt.
We've never used boars that young for breeding and if i'm honest i'm not sure about their age of maturity for breeding. Ours are normally 10 months before we place them with gilts and second parities sows.
Maybe others on here can offer more advice