Mystery Challenge Number 4
"I was faced with a modest young lady the other day," Maxwell said to Granny. "She was chary of confessing her true age."
"As many women are," Granny noted. Then under her breath whispered, "Me included!"
"Quite so. I was reasonably persuasive on the matter however, and finally got her to admit that she was the eldest of fifteen children, each born with a year and a half between them. When she confessed that her age was 8X that of the youngest of her siblings, I knew at once how old she was."
How old was she?
With 21 years between the eldest and youngest sibling, the girl must be 24, and her brother just 3.