How well supported is the hypothesis that aging is mainly caused by the number of mutations in tissues?
I think telomere shortening largely accounts for the replicative cell aging.
http://www.ncbi.nlm.nih.gov/pubmed/22901253
The increase in no. of mutations with age surely does affect the tissue/organism survival too. There's a recent study that I'd like to share here. It is quite interesting and informative.
http://www.iflscience.com/health-and-medicine/mutations-found-115-year-old-womans-blood-could-help-unlock-secrets-aging