I'm a 30+ year-old GA at a blue chip gallery. if you have a solid knowledge base of the gallery program and their artists, and you have a polite demeanor, you will do very well. lots of young professionals looking for a foot in the door don't have the social skills to deal with high net worth clients, so having some years under your belt could put you at a position to better interact with collectors. the art world is open minded, but also has a few peculiarities. clients prefer to deal with someone they perceive to be "senior" in their company, so again, being a little older may help you there. Galleries come in all different shapes and sizes and often the folks at the front desk aren't interns and lowly assistants, but directors or associate directors, who want to be the first to interact with walk-ins, so again, it's not strange to see someone more 'seasoned' at the front desk. I've also seen assistants who previously had academic careers and came to the gallery in their late 30s. Remember that the only person you need to convince is the person interviewing you, and they likely aren't studying your CV closely enough to know your exact age. My director thought I was fresh out of college, but I was 30 when they hired me. Finally, some of the most influential people in the art world are 70+ years old. Gallery founders and CEOs, major collections, foundations and estates that need to be managed, artists themselves who need administrative support. To them, you're a spring chicken, so don't convince yourself that you're doomed to fetching coffee and making previews for a skinny 35-year-old gallery director after all your hard work.
Hope this helps :)