Adding function to calculate years since a reference timestamp
- [ ] closes #xxxx
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Why is this needed? I'm not sure that "need" is the right standard to use here.

The common use case is calculating a person's age from date of birth. There are numerous questions of this type out on Stack Overflow. Most answers suggest using a time delta and dividing by 365.25, but this is erroneous due to leap years.

A sampling of Stack Overflow questions on the topic:
- https://stackoverflow.com/questions/26788854/pandas-get-the-age-from-a-date-example-date-of-birth
- https://stackoverflow.com/questions/26897098/pandas-calculating-age-from-a-date
- https://stackoverflow.com/questions/51491893/how-do-i-get-an-age-in-years-and-date-on-pandas
- https://stackoverflow.com/questions/46508895/calculating-age-from-date-time-format-in-python-pandas
- https://stackoverflow.com/questions/49726125/how-to-calculate-age-based-on-date-in-python-pandas-data-types-errors
- https://stackoverflow.com/questions/56170259/calculate-age-in-a-pandas-dataframe
- https://stackoverflow.com/questions/49819706/how-to-get-an-age-column-through-date-of-birth-in-pandas
- https://stackoverflow.com/questions/59005559/how-to-calculate-age-from-date-of-birth-in-pandas-pythonjupyter-notebook Yea I'm also not sure this is useful enough to have a dedicated method for - is there an open issue / discussion for this? agree here -1 on extending this api  Thanks @bryan-woods for the PR. Unfortunately I don't think the core team has an appetite for this method, so going to close this for now. 

If it's something you feel strongly about I'd suggest opening an issue for discussion first and can reassess from there