It's probably fair to say the term is more common on earlier certificates and that creates problems
a "full age" marriage cert from 1842 for example would indicate a birth from around 1821 or earlier but it isn't a gimme, and someone born in 1821 could easily be baptised many years after their birth (if at all).
Proving relationships and delving backwards sometimes involves accumulating several other sources of information such as age on subsequent census, age at re-marriage if applicable, age at death registration, references made to the individuals and their siblings/children in probate records, parish relief records etc.
