Approximated birth & marriage date can be close & correct, but this always needs to be double checked from the sources available. The rule of thumb given by the LDS FHL is that males get married at about the age of 25, females about the age of 21. Then, typically the first child is born 1 year after the marriage. Each addition children are born about 2 years apart.

As you know, this does not always fit! It can be difficult without CMB records. I have seen ages given in Presentments & Composition books listed in eras where there is no surviving Parish Registers. I still also wonder if some of the lost Manx Bishop's Transcripts are in York's Diocese Records.