I would say 'I would never have a child at a late age' or 'I would never have a child later in life'.
'at an old age' is okay, but it makes me think of a great age, like 70, when it would be pretty obvious that you wouldn't be having a child.
I'd also say you wouldn't 'have a child' rather than 'give birth to a child' because the latter makes me think of your suddenly deciding not to give birth at the last minute after having been pregnant, which obviously isn't possible :D If you decide not to 'have a child', you decide before the child is conceived.