agree, 18mths can start. you can try once per week first, if he/she likes, then can join more classes. 2 hours/session is ok, 1 hr too short and 3 hrs too long for them is my opinion.
yes, my son about 18mths and now enrol some classes during weekday (1 class) and weekend (1 class) so that I can play with him together during weekend.