Hi Veronica!

I also teach in Spain, at an academy, but there is still a huge demand locally for private lessons for all ages!

I would say that the age of your students shouldn't be too much of an issue so long as there are no huge gaps, say one of 5 and one of 15! So long as they are of a similar age group, tailoring individual private lessons that are suitable for all 3 is very much doable. The bigger question would be what ability level are they? If there is significant difference between their use and understanding of English, then private lesson may flounder somewhat.

Ability is definitely more of an indicator than age (excluding large age gaps, of course)