The decline in functionality begins to be detectable between 20-40, and accelerates with age. This study has a number of measures regarding bone mesenchymal stem cells (which are included in what we collect and store):

https://www.ncbi.nlm.nih.gov/pubmed/18248663

Ideally, we all store at 18. However, my mother just stored at 70. Her mother lived to 93, and two of her aunts over 100. She decided that in 10 years, she might be glad she did. My own cells are 40. I am 42.