Usually to be considered a part of another person's generation you have to be born within a 12 year time window of that person, more or less. But the formation of the egg a person 'hatched' out of is a factor to consider.
Remember that all the eggs a woman carries in her life form a few months before she is born. What this means is that a Millennial, Generation X, and Baby Boomer may be from different generations but they can be of the same cellular generation.
If their mothers were born in the same month and year, then that means all of their first cells formed at nearly the same time of the year.
Perhaps the oldest most famous example is the King of Siam/Thailand and the Queen of England/Great Britain. Both of their eggs formed in the year 1900. Coincidentally both their first cells are at zero points of genesis.