I would not say they are the only cause of aging but short telomeres certainly contribute. Short telomeres encourage gene expression that promotes inflammation and senescence not only in themselves but in adjacent cells via sasp.
Longer telomeres offer protection and encourage youthful gene expression via the tpe effect aka telomere positioning effect.
Longer telomeres promote a similar level of youthful expression via tpe over long distances aka tpe-old. Almost like the reverse of sasp and how it influences nearby cells.
Short telomeres are not the sole driver of aging but i believe they are acting not only as a cell division clock but also as a gene expression regulator that determines what genes to express based on its length. In other words it’s length tells it how old it should be and this influences the age of its local environment. This also may explain how some organs age faster in some people than others, some local telomere populations are wearing out sooner than others likely due to more exposure to stressors.
If you reset telomere length you should see restoration of tissue and I believe a return to youthful gene expression. This restoration has been observed in animal studies eg, blasco et al.
I believe telomeres are contributing to aging via gene expression. They are not the only cause of aging but they are one of them which makes them a primary driver of aging as they cause so much damage when they shorten.
Big reason they should be targeted as a priority