My 15 month old baby is exclusively breastfed and no signs of weaning. When is the ideal age for her to wean?

There is no "ideal" age for baby to self weaning. It all depends on how comfortable between you and your child. I know children bf all the way up to 3 to 5 years old with excellent immunity. I bf my firstborn until 28th month and she self-weaned off as I am pregnant with our second child. However, she still prefers my milk until now at (going to be) 5.