Igros Moshe (O.C. 02:097): Bas Mitzva and bar mitzva. This that you question why isn’t there a mitzva to make a meal for a bas mitzva when a girl is 12 and becomes obligated in mitzvos as there is there is for a boy when he is 13? Let me reinforce the question. Even though the girl does not give a Torah speech and even if she does – it has no significance since she is exempt from Torah study, nevertheless on the day of a bar mitzva it is considered a seudas mitzva even if he doesn’t give a Torah speech (Magen Avraham 225.4 and Yam shel Shlomo).So similarly it should also be for a bas mitzva that on the day she becomes 12 and one day that it should be considered a seudas mitzva. Look at the Pri Magadim who asks why isn’t the beracha (Baruch Shepatroni) said when a girl becomes 12 but does not ask why isn’t a meal made for her even though the reason for the meal is because it is greater to be obligated when you do mitzvos which also is relevant for girls. A possible explanation is because the change is not so obvious when a girl becomes an adult when compared to a boy who is now counted as part of a minyan or where three men are required. So with the knowledge of a change which is not so obvious we don’t make a seudas mitzva and celebration.