Is a therapy designed to treat mitochondrial disease setting its sights on age?

Mitochondria provide the power for our cells, and when they malfunction, it can be serious. So scientists have developed experimental treatments that use healthy mitochondria from donor eggs. 

In one variation, the nuclear DNA is removed from a donor egg and replaced with the nuclear DNA from an intended mother's egg; the donor egg's mitochondria, left behind in the cytoplasm, provide the power pack. The egg is then fertilized. In another variation, both an egg from a donor and an egg from the mother are fertilized before that swap takes place. Almost all of a person's DNA is in the nucleus, and almost all traits are determined by this nuclear DNA, but mitochondria also contain a small amount of DNA. This means that any child born from such a therapy will inherit DNA from three people — the mother, the father and the donor — which has given rise to terms like 'three-person IVF' and 'three-parent baby.'

The treatments have been controversial since the outset, because it's not yet known whether there will be unintended side effects. It's not easy...