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

In my experience, I don't let my babies wean. My eldest is almost 4 years old and still no signs of weaning so I let him be. My youngest is 19 months so I still tandem feed them.

My daughter is also 15 months old and still breastfeeding. I plan to continue breastfeeding her for as long as she wants.