Baptismal Dates are Baptismal Dates

Always indicate that dates of baptism are dates of baptism.

In churches that practiced infant baptism, dates of baptism are usually "close" to the date of birth, but do not assume that every baptism can be used to approximate the date of birth. Some children may be baptized as toddlers or even young adults, depending upon the availability of a minister, the  strength of the parents' convictions, etc.

If parents are having one child baptized every two years over a fifteen year time span, then probably each one is being baptized as a small child.