If I see a young woman with a baby, do I ask" ¿Cuantos meses tiene?"
Si veo a una joven con un bebé, ¿pregunto?"¿Qué edad tiene?"
I’m sure both of those would be understood…
@Deborah-SSi @gabycortinas any other common usage patterns you’d suggest?
FWIW, I would use both of those but if the child was clearly only a few months old I would probably not ask about his/her edad.
The most common phrase I would use is ¿Cuántos meses tiene? and occasionally I would say ¿Cuántos meses tiene ya? the “ya” at the end is simply “now”, but like in English, just adds a touch of emphasis (ie like in How old is he? VS How old is he now?
The other phrase you might hear in some places is just simply ¿Cuándo nació [èl niño/la niña/el bebé/etc]? (ie When was the child/baby born?). Although this is more common if the baby is a newborn, I have certainly heard Southern Mexicans ask this about toddlers!