When I was studying dance as a child, there was a rule that students could not begin pointe work until they turned twelve. I remember that I could not wait and even had dreams that there was a medical reason that I had to start earlier.
After researching the question: When is the appropriate age to start pointe work, I found that it is alright to start before the age of twelve. In fact, age has nothing to do with being ready. It depends on how strong the student is and if they have proper placement when performing ballet steps.
This makes sense to me since children mature at different rates. Also, each body is unique. Some students are naturally more flexible and some are stronger. In addition, many dancers take multiple classes per week, are more focused, and may have studied for several years. Others take class on a recreational basis, so they are not as strong. So, age should not be the indicator of pointe work readiness.
And, it is fine for adults to do pointe work. Of course they must study ballet regularly and have proper alignment and technique.
Here are some of the books and articles that I used for my research:
The Pointe Book, Shoes Training & Technique
Perfect Pointe Book (strengthening exercises and tests to see if the student is ready for pointe work)
The Healthy Dancer, ABT Guidelines for Dancer Health
Too Old for Pointe? (Blog by Debra Vogel)