Agreed I dont ever have any clue really how old my plants are or even how long they've been in flower. I mean I know within like a week or a half a week but they can act so differently from strain to strian/grow to grow that I just pay attention to how big the plant is, how developed the buds/hairs/trichs are etc.
I mean if they've been in a cloner for that long I can't imagine they've grown much (if at all--depends on if there were nutrients). I personally like to veg the shit out of my plants, its a lot easier getting more weight by vegging for even an extra week or two. So I would probably veg them for at least 2-3 weeks before I even thought of flowering.