Estimating Date-of-Birth (DOB) to identify individuals to within realistic time zones. I have found the following rule-of-thumb provides a good starting point in which to start searches. 

As a first estimate take the same DOB as that for the spouse. Make variations to this when a mothers 1st child would be after she was 30 years old. Also take the age of parents and children into account. A general rule is that 1st children are born when the parents are about 25 years old and 1st marriages usually occur a couple of years before that.

Other considerations

For England and Wales, in times past apprentices were not allowed to marry - and all persons under the age of 21 years required parents permission - or that of a Court. That age is now 18 years.

Again for England and Wales up to 1763 - the legal age of consent was - 12 years old for girls (sometimes younger) and 14 years old for boys. From that year the age was raised to 16 years for both sexes and has remained so to this day.

I am indebted to John Chapman for providing corrected dates. More about dates and registers here.

As you get back further - it was the custom for younger sisters NOT to marry before the marriage of any elder sister.

Civil registration started 1837 - but it was 1868 before civil birth & death registration was required by law. So if you fail to find someone - check church records.

Whilst parish registers in England and Wales were formally instituted by Act of Parliament in 1597 - a few go back another 60 years.

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