Ditch the hats, ditch the T-shirts, wear something button up, nice jeans or khakis/cargos, and don't wear tennis shoes. Should add a couple years.
I know your pain with the age thing. Am 28 and the oldest I ever look is about 20-ish. Which I used to not mind, but considering my bf is in his 50s and our ages are far enough apart in reality as it IS, I don't need to appear to make that gap any wider. People stare sometimes, especially since he looks a bit older than he actually is. So rather than our true 28-year difference, it can look closer to 40.
I don't care when it comes to us, but like I said the age span does draw some eyes sometimes and that can feel uncomfortable.