I attended a special Mass with a ceremony that my aunt (my dad's brother's wife) was a honored participant with about 30 others (commissioning as a lay minister). The church was pretty filled up with family and friends of the honorees. The whole Mass with the ceremony lasted about 2 hours total at the diocesan Cathedral.
Behind me where I was seated with my parents were about 3 or 4 kids all between age 3 to 7. They kept getting up, running around, talking loud at times, making noises etc and there were 2 adults with them. I turned around several times and placed my finger against my mouth as a signal for them to be quiet. I know I should have not done so but I wanted to hear the service/ceremony as I was also doing non flash photography in the front row of the balcony to capture my aunt's important day (as her husband was in the choir and could not do so).
I would say all of them were too young to be there for a service that long. I say unless a child knows better to stay seated and reasonably quiet, they should not be there (unless a parent cannot make arrangements for child care). I am not against kids but an important event that my aunt was a part of, it was important for others in the same area to be able to hear and see things without much interruption.
My best guess is for a child about 6 or 7 years can behave decently at something that long even if they have a quiet toy and/or book to entertain them at times. The few babies that were there I rarely heard them as they were either removed at the first sign of being fussy or were already in the cry room.
Please keep in mind, I am not against children, and that Jesus welcomed the youngsters when they were brought to them & He said we should welcome them. But I am sure they did their best to be quiet when he told them a story too.