These days a fair amount of scientific work is aimed at quantifying the benefits of various different approaches to exercise. The research here is an example of the type, and compares endurance training (aerobic activity) versus resistance training (to build strength). The authors looked at measures of telomerase activity and telomere length in white blood cells obtained from a few hundred volunteers who carried out different programs of training. Some groups showed greater gains than others.
This should not be taken as robust evidence for effects on aging, as firstly this is more an assessment of immune system activity than of the state of the body as a whole, and secondly telomere length is a truly terrible measure of aging. It correlates very poorly with aging in all but the largest groups. All this really tells us is that aerobic activity fires up the immune system more readily than resistance training. It is known that both aerobic and resistance exercise affect aging, and in different ways, but this study isn’t the way to usefully quantify those influences.