Some people seem to get very pernickety about ages in their family history research. This is not really justified. There are many reasons why ages in historical records may not appear to tally exactly with what you might expect. Never reject a record just because the age isn't exactly right.
The first thing to consider is what you are using as the persons birth date, since this obviously defines what you expect their age to be at some later date. If you are lucky or have obtained a birth certificate, or a modern death certificate, you may well have an exact birth date. Be aware, though, that whatever the source of this information, there is no absolute guarantee that it is correct. In the absence of an exact date then for England and Wales births since 1837 the usual source of a birth date is the General Register Office (GRO) registration index. This shows which quarter of which year the birth was registered. A birth was supposed to be registered within 6 weeks, but this did not always happen and sometimes the registration was later. In some cases when a birth was registered late, the parents gave a date less than 6 weeks earlier as the official date of birth in order to avoid getting a fine. There are also some circumstances in which a birth may be re-registered, e.g. adoption or subsequent marriage of the parents. This may happen years after the birth.
In general the actual date of birth will be in the given quarter year, or the the immediately preceding quarter. In rare cases it may be earlier.
The other common source of a birth date is in parish records as a baptism. Occasionally a baptism record also gives a birth date, but usually this is not the case. If it doesn't then all that we can deduce is that the birth must have been before the baptism. How long before is a moot point. Children were usually baptised within a few weeks of birth, but this was not universally the case. Sometimes baptisms would be left for years or not happen at all. If a vicar discovered that someone he was marrying wasn't baptised he might insist it was performed before the marriage could happen.
The birth date is generally a little while before the baptism but in some cases could be years earlier.
The records where ages cause the most headaches are the censuses. In these the stated ages may not be accurate. This is often because people are fallible and may forget exactly how old they are, or their children are. In the 1841 census there is the added complication that the enumerators were told to record the age rounded down to the nearest multiple of 5, for people over the age of 15. Thus a 23 year old would be recorded as 20, and a 59 year old as 55. But not all of the enumerators understood these instructions; some rounded to the nearest 5 years and many simply provided the actual ages. The fact that the censuses only record ages also means that the researcher has to deduce the birth year from the person's age and the date of the census. Subtract the age from the census year and you have the birth year, don't you? Well, actually this is only true if the person has already had their birthday for this year, otherwise the birth year will be the year before this. For example the 1871 census was taken on 2nd April, therefore if someone was 30, then they appear to have been born born in 1841. This true if their birth date was 2nd April or earlier, but if they have not yet had their birthday then the birth year will have been 1840. Note that the censuses were usually taken nearer the start of the year so the latter is more often the case.
For reference the dates of the available censuses were:
- 1841 7th June
- 1851 30th March
- 1861 7th April
- 1871 2nd April
- 1881 3rd April
- 1891 5th April
- 1901 31st March
- 1911 2nd April
Another potential complication with dates is the change from the Julian calendar to the Gregorian calendar. The difference between the two calendars is the way that leap years are determined. The Julian calendar was less well aligned with the true length of the year so gradually drifted away from an accurate date. It's year was slightly too long and so when the change was implemented a number of days had to be skipped to bring the calendar back into line with reality. In the UK the change was made in 1752 and 11 days had to be skipped. Wednesday 2nd September was therefore followed by Thursday 14th September. In the same year the UK adopted the New Style year numbering. In the Old Style prior to 1752, the new year started on Lady Day, 25th March, whereas in the New Style it started on the 1st January. (see here for more about dates). Be vigilant, the change in the start of the year was made at different times in different countries, as was the change from the Gregorian calendar to the Julian. In Scotland, for example, the year had started on the 1st of January from 1600.
So be careful when computing birth dates from ages, or when computing a person's expected age at a given date. It may not be as simple as you expect even, if all your data is accurate. And errors are always possible in the data, or at any later stage.