I've always wondered about this, but how old is Hiei?

Hiei MUST be under 100 years old, because his twin sister Yukina is a koorime/ice maiden, and they reproduce through parthenogic conception/placental self-cloning every 100 years. Yukina has no children, so both twins are under 100 years old. Also, Yukina is an exact genetic duplicate of their mother Hina, and Hina (in the flashbacks) LOOKS OLDER THAN YUKINA. This shouldn't be overlooked, since Yukina fully grown will look exactly like her genetic predecessor. Most people quit growing at 21, so the twins are under 21- but Hiei has a low, past-puberty voice and hasn't grown noticably during the 3+ years time span of the series- so, I'd place them both at between 15 and 18.

His Age is never revealed in the anime of manga
But he looks like he is in his prime but for a demon he might be older probably 100+ years old and that's my answer.

between the age of yusuke and kuwabara

Old age?