La población de Japón es mayor que la de Inglaterra y Francia juntas.

English Translation

Japan’s population is larger than that of Britain and France put together.

Could this also mean that the population of Japan is older than that of Britain or France?

Without juntas I think the sentence is ambiguous in the way you’ve described. But with juntas it sounds like you’re saying the average age of Japan’s population is larger than the sum of the average age of England’s population and that of France’s population. That could be an alternative translation, but it doesn’t match the “or” translation.

Thanks for your reply. I was thinking that “mayor” usually dealt with age, but now I realize that there are many other translations for it.