Typically closes around 12 months of age. Can be bulging (meningitis?), sunken (dehydration), pulsatile (normal!), large (hypothyroidism), small (craniosynostosis). But actually not very predictive of any of these things in isolation.

For example – in Brazilian study of babies with craniosynostosis compared with babies with fontanelle that closed by 6 months, only 36% sensitive for craniosynostosis, and positive predictive value 59%. [https://doi.org/10.1016/j.jped.2021.10.004]