You're trapped. A long elevator ride. Ferris wheel. Bus stop. Coffee shop. Or in my case, a wine tour shuttle. And the drunk old lady sitting behind me and my new bride was busy telling us how we could preserve our marriage for years to come.
Unlike most twenty-somethings, I don't mind a good bit of sagey wisdom from a crotchety spinster or grayhead. I love hearing them talk, love the pitch of their voice as they take command of their memory, dredging up some life example from their own past that applies to this situation, this moment, the moment they think applies to you. Taking whippersnappers and jackanapes to task or unburdening their own weighted wisdom upon you is the province of the aged, and youth are douchey enough to warrant it most of the time.
However, like nuclear disarmament or expiration dates on milk and eggs, one has to draw a line somewhere. Blanket acceptance of the Infirm Advice Quotient leads to stretching one's patience into inhuman dimensions, the invention of new non-specific head-nods to indicate non-specific agreement, and eventually the unfortunate but sadly necessary awkward smile as you slowly back away. The unaffiliated aged will wax on and on, pulling freely from stories that do not apply, their hands sometimes shaking as they wrestle the past into submission for this catch of youth who isn't impolite enough to run away.
Grandparents, like the devil you know, are "Persons of Years" who possess familial relations and thus carry relevant data about you, from which they can form opinions and therefore offer critical advice to you, advice that doesn't bear the stench of generic platitudes pulled from the aisles of Target's Hallmark and Papyrus selections. Similarly, the elderly who you know in passing, from interactions in public spheres, and adult friends of friends are equally qualified, however distantly, to dispense their cogitations and corrective axioms.
The problem is not that senior citizens give advice. It's when they give advice freely, unsolicited, to absolute strangers to whom they neither owe nor qualify for giving advice to, that the problem, like a Cuban in a Cadillac, floats your way. In those cases, feel free to fire back with some spry advice of your own; its not as if they were never young once.