Use it or lose it.
If you stop learning you forget how to learn, it sounds stupid but it seems to happen. The other thing is simply as you age there is less to learn. When you were younger, everything was new and interesting, that isn't the case as you settle into life and a job, which is right about your age. You settle in and get comfortable. When was the last time you learned something truly new that took you outside your comfort zone? Don't get complacent.
Don't play brain games games, don't just get a hobby, get a hobby that forces you to learn. Learn cad, 3d printing, model rocketry, programming, art, magic, pottery, music, acting, comedy, cycling... anything that challenges you and requires actual learning a skill, be it technical, physical or artistic. While not a great time to do so at the moment, local hacker spaces and makerspaces are a great place to get your creative juices flowing.