My granddaughter is 17 months old but her anterior fontanelle is still not closed. Why?

Hello doctor,

My granddaughter is 17 months and 1-week-old. She was a term baby born out of a normal vaginal delivery. She weighed 6.61 lbs during birth and was 6.17 lbs when we were discharged from the hospital four days later. She was exclusively breastfed for six months, after which we introduced solid foods (in addition to breast milk, which is still continuing). She is a very bad eater, however, we are managing to feed her protein-rich food thrice daily using a lot of diversion tactics.

My questions are: Why her anterior fontanelle is still not closed? Still, there is a little bit of space left. She seems otherwise healthy and active, and seems very intelligent in the way she grasps things through books, remembering every single thing. Her weight also seems normal, although she is not overweight or chubby. Is it normal for the fontanelle to still remain open a little bit? I know it is supposed to close completely by 18 months, but I am wondering if it will close in the remaining three weeks. Suppose it takes longer than 18 months to close, is it fine? Please let me know.

Thank you doctor,

I would still like to know, up to what age maximum we can wait for the fontanelle to close?