Depends on the church - some churches won't do baptisms until the child is of an age to actually understand what it is that they are doing. Sort of like an informed choice, not pressured by anyone else. Others will do christenings or baptism from birth on up. Just depends on the tradition one is a member of.
In the Catholic tradition, babies can be baptized anytime from immediately after birth. Contemporary Catholic theology is waiting until the child has reached the age of reason (around age 7) so they canap? that decision for themselves. My wife and I baptized our children soon after birth, but let them decide about confirmation. Only one practices our faith.
I was six. A child needs to be old enough to make the choice themselves and understand what they are doing and why. It has to be a personal choice, not forced upon you or it doesnt count. The age of understanding is different for all people. I understood well at age six and told my parents i was ready. Some oeople arent ready until adulthood. I HAS to be of a persons own free choice tho, not done to them when they are too young to even realize whats going on. God is all about free will. The catholuc church is far far too much into taking that free choice away and trying to force things on people.
When the child is at the age of accountability( to understand right from wrong/good from evil) usually 9- up( depending on the maturaty of the child). Baptising babies is only symbolic that the parents are promiseing openly to declare to bring up( train up) a child in the understanding of scripture! Baptism is a showing to the public that you have put your faith in Jesus /and symbolizes your death/burial and resurection WITH Christ! A New person- and dead to your old self(ways-attitude-sinful ways)