Most parents know the phrase “18 summers.” It’s that gentle reminder that you only get 18 summers with your kids before they’re grown and gone — 18 chances to build sandcastles, take road trips, and watch fireworks together. It’s heartwarming… and a little heartbreaking. Because once those summers are gone, they’re gone. But here’s something most people never think about:You get another version of “18 summers” later in life — during retirement. I’m not talking about how long you’ll live, but how long you’ll stay healthy and active enough to do the things you’ve dreamed of. These are your go-go years — the years when you can hike the Grand