It's less of them being mature and more of the fact that living in a community is really difficult and takes a lot of adjustment. People that are older (most communities are cut-off at 35, but it's not uncommon to go higher like 40-50) become a lot more comfortable with how they had been living their lives and being able to choose what they want to do most of the time. You literally take a vow of obedience as a nun, so it's a lot of adjustment that a lot of older people just can't make. Being a nun isn't just waking up early to pray and doing your teaching or ministry during the day. It's living a specific way of life that you can't really take a step away from.

I'm sure she had good intentions, but the community must have thought she just wasn't ready. Hard thing to see, especially for people that want that stability of life.

Not necessarily. There were two "types" of monks: choir and lay monks. The choir monks were usually priests (male monastic communities select which of them become priests) and would spend most of their time in the choir, literally singing the prayers of the community (big deal in monastic setting). These were usually the wealthier people and they were often physically separate from the lay members, who were not taught Latin (and thus couldn't sing in the choir) and were responsible for the physical upkeep and labor of the monastery. These usually went to members who did not have any familial status or wealth. So people could join, they just were often tasked with the manual labor and less glamorous jobs.