Nobody is criticising her because she is engaged with whatever issues she is currently working through.
ANY 17 year old who is engaged to the person they had been seeing for 5 months, any 17 year old period, would get the exact same response, and rightly so.
If (and I stress if, because I would bet my left nut that they are just drunk on teenage 'love', at best) they are truely mature enough and experienced enough to know that they want to make such a commitment to one another, waiting until they are older to go through with it wouldn't matter in the end.