zoro would have been 1 year old (or a newborn if you count age in one piece the same way as in Japan, where newborn babies are 1 year old) when the daimyo rebellion failed. ushimaru's age isnt known, but hes old enough to be a father in the yamato flashback. zoro could have easily been born in wano and then smuggled out after the daimyo rebellion failed.
if you want to be ignorant on facts go for it