I was 12 when I began wearing contact lenses. In those days, all we had were hard lenses, and we weren't allowed to wear them to school until high school (10th grade for us--9th grade was part of "junior high"). Not sure what they thought we would do, but those were the rules!
At age 12 I was prescribed reading glasses along with my distance vision glasses--or bifocals. Never could adapt to the bifocals, so my parents let me try contact lenses. I also was involved in intensively in ballet, so the contacts allowed me to see while rehearsing and performing. Glasses were okay for class, but they didn't work well in rehearsals and were impossible for performing!
My optometrist and his assistant were terrific in working with me. They drilled the care and feeding of contact lenses into my head very well! To this day, I always wash my hands thoroughly--including under the nails--before inserting or removing my contact lenses or handling them for any reason. *LOL* Others laugh at me, but I never have had an eye infection or scratched a lens. Lost one, yes. And they do become worn (RGP--rigid gas permeable) and scratched over time, but usually I can wear a pair of lenses for two years--thankfully my prescription doesn't change very much. Then I purchase a new pair and the old ones become "back-ups" that I wear about one week per month--just in case I ever lose one of the new ones!
Everyone is different. Most of the young people I know today get their contact lenses in high school. Most are not sufficiently mature to handle them before age 15 or so.
Edited to add:
Apart from back-up glasses, it's also necessary to learn to allow a bit of extra time in the morning for the contact lenses. My BFF gets very annoyed with me, as I allow about 30 minutes to insert my lenses in the morning and allow my eyes to adjust to them. Not everyone needs to do this, but it's a good idea to plan for the contingency. My BFF is my travel buddy and just jumps out of bed ready to rock and roll when we're on holiday! *LOL*