Ref. SarahofCrantock - "That is interesting, what makes you think the dates are invented?"
These dates - 1554 for the birth of John Martyn and 1558 for the birth of Elyzabeth Nankevall - are unlikely to be from baptism records, because (1) records for St C Min only seem to start in 1560 (according to OPC Cornwall site) and (2) they don't give day and month. So if they are true, they probably come from some source (marriage, burial, will) that gives age in years; that also seems unlikely - I haven't come across examples in 16th century Cornish marriage or burial registers though perhaps there are some.
So the alternative explanation, that they're invented by a family historian, seems much more likely.
I've seen lots of examples of that, and basing them on the woman's age at marriage as 21 and the man's as 25 is common.
It's a very annoying habit, because on the occasions where the bride and groom's ages at marriage are genuinely given (as is fairly common in nineteenth century registers) it can be very useful information.