Say a State changes its drinking age from 18-years old and moves it to 21-years old - people aged 18, 19, and 20 would be "grandfathered" and not subject to the new 21-year old drinking age. People 17-years of age? Well they're screwed.
The term "grandfathered" has become well known and pretty benign but the origin of the term "grandfathered" has some serious racist overtones.
After the Civil War many of the Southern States instituted some new voting restrictions aimed at making it difficult for newly freed slaves to vote. Poll taxes, literacy tests and other nefarious rules were put in place but these new requirements had to also apply to poor whites. How to get around these new rules then? Add another one that said "if your grandfather could vote so can you". Of course prior to the Civil War only whites could vote and if your grandfather could vote - that means you're white too and thus it's A-OK for you to vote.
That's how the term "grandfathered" was born. As I said before the term is pretty benign now and at some point you have to just accept your past and move on but honestly now knowing the origin of "grandfathered" will make me second guess using the term myself.