Did you know that as soon as you hit 25 you officially start aging? Growth stops and body processes start to slow down. Short-term memory degrades, synapses don’t fire as fast and you get fat. Your body will never work as well again as it does now — you’ll never be as energetic, as athletic, as sexually active. You turn 26 and your youth is scientifically over, gone, kaput.

Not a pleasant thought, but luckily most of us have at least a few years left before this gentle slope downward to death begins. For the most part, it’s easy to avoid thinking about impending oldness — we’re at college, after all, where the only people over the age of 23 are professors and the frustrating clumps of grad students taking up all the tables at The Publick Cup. Here at Yale, youth reigns supreme.

At the Long Wharf Theater’s world premiere production of “Lil’s 90th: A Late-Life Love Story,” though, a different age group dominates. The title of the play, written by Darci Picoult and directed by Jo Bonney, is a good indicator of the audience demographic. On the night I went to see “Lil’s 90th,” I was at least 40 years younger than any other person there. I felt distinctly out of place and as the play began, it became clear that there was a reason why the seats were filled with members of the AARP.

“Lil’s 90th” is billed as a love story, but it is also unabashedly about old age. The plot revolves around Lil (played by actress Lois Smith of “True Blood”) and her plan to throw herself and “everyone who’s not dead” a massive cabaret for her 90th birthday. Lil is immediately likeable — she’s enjoying the kind of old age everyone wishes they could have, complete with grandchildren, a loving husband and undiminished enthusiasm for shopping, singing and performing. As the play goes on, though, an unsettling counterpoint to this ideal emerges. Charlie, Lil’s husband (played by David Margulies, in what is easily the play’s most moving performance) represents the fears we all harbor about old age: He’s a sweet, wonderful man, but it soon becomes clear that he’s developing some debilitating form of dementia.

While Lil rehearses for her “debut performance” and goes dress shopping with her daughter, Charlie starts to slip further away. His secret phone calls and assertions that he’s “come into some money” are unsettling from the start. Those actions, already strange, are set alongside Charlie’s inability to remember the names of people he meets and his tendency to repeat himself again and again when telling stories. When the truth of what Charlie’s done with the family’s money is finally revealed, it’s exactly what you expect — and it’s terrifying, because it’s an example of the ultimate loss of control from a man who was once the breadwinner for his small family.

The other three characters in “Lil’s 90th” are Lil and Charlie’s daughter, their grandson, and his girlfriend (played by Kristine Nielsen, Nick Balemire and Lucy Walters respectively). The young couple is particularly interesting, because their hesitant relationship is a moving example of beginning in a love story that is all about endings.

In fact, the most deeply uncomfortable part of the play is not what Charlie does; it’s Lil’s desperate refusal to accept the reality of what he has become. Throughout their arguments and screaming fights, Lil and Charlie clearly remain deeply, irreversibly in love with one another, which makes it all the more difficult to come to terms with the growing inequality in a once balanced partnership. The play’s final scene thrusts Lil and Charlie into the roles they will have going forward. The ending, though hopeful, is still tinged with a sense of bitter inevitability.

For the most part, the acting — particularly that of Smith and Margulies — is superb and the performance I went to received a standing ovation. For me, that was the most interesting part: not the play itself, but the reaction of the audience. The woman next to me, who literally had an oxygen tube in her nose, got out of her chair to applaud at the end and the two white-haired men in the row in front of me were dancing along to the music that played while the actors took their bows.

Scientifically, the older you get, the happier you get. You spend less time thinking about the now-not-at-all-distant future and more time enjoying things in the present — your family, for instance, or maybe a play. For all that “Lil’s 90th” concerned itself with endings, it was also about finding happiness in the present. It revolves around Lil’s insistence not to let anything get in the way of her big performance, her gift to herself. We get older, things go wrong, but like the show, life goes on. You may as well enjoy it.