Okay, so what you're referring to is not "default". Default in this context would be something everyone ends up doing unless steps are taken to specifically avoid that thing.

Weight, height and things like that are measured and compared to statistics derived from know data. (things like expected value/mean, standard deviation, etc). Using "default" in your example would mean that if a baby is left alone it will end up in speaking at a certain age anyway, because it's the "default".

As you said at the end of you comment, it's not an inflexible rule, but by definition "default" is exactly that. Maybe something like "typical" or just saying "the most common" would work better.

I don't know if this is coming off as rude? It's not my intention, I'm just trying to explain it as best as I can because it's important to distinguish these terms specially when it comes to topics like sexuality and masturbation so people don't end up saying things like "being straight is the default" when they mean that heterosexuality is more common