I dont know how freshers works exactly but ive been out in Aberdeen before and you wont get into any bars or clubs without proper ID, and yes they do sometimes ask for two.
It may be the case that if your flatmates are drinking and have pre-drinks in the flat you may be able to drink with them as long as they're willing to risk with supplying you with alcohol, though you'll also have to be wary of anyone monitoring the halls.
Its not the end of the world though from what I gather there'll be events on during the day at freshers which you can go to with your flatmates to get to know them that don't involve drinking.
I know it probably feels kind of sucky but you willl be able to join in after Christmas and you can do freshers again next year as well. Its may not be worth the trouble you can get into trying to sneak into places underage or the trouble others could get into for supplying someone whose underage with alcohol.