Till what age can an infant be called akachan. When can we use musuko-chan till any particular age or till he is able to walk. Please clarify
”Akachan” is ”baby.” The meaning of it has not far from born and depending mother. If he walk stable or talk, or you talk with one who thinks he is not a baby, you cannot use it.
On the other hand, you can use ”musuko” even if he is a baby. But, ”-chan” is not suited. If you talk about other’s son or daughter, you can use ”musuko-san” or ”musume-san, ” or simply ”oko-san.”
赤ちゃん (aka-chan) is a familiar derivative of 赤ん坊 (akanbou). 赤ちゃん (aka-chan) is only used for one's own child.
According this dictionnary entry it is said that 赤ん坊 is a fetus, a newborn or an infant. An infant being a child less than 18 months old or that cannot not walk nor talk. It is fairly close to the meaning of the English word "baby".
The suffix -ちゃん (-chan) can be used on 息子 (musuko) and 娘 (musume) for one's own child or the child of a familly member. However, note that this usage is not appropriate for someone else's child. It should be preferably used for newborns and infants.