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.