Hiking is one of those sports you can pretty much be any age to engage in. Old or young you can stroll or climb your way through the forest. Hiking does more than get you in shape. It also gets you out in nature where you can unwind from hectic life. It brings you back toward a simple time when doing nothing but enjoying nature was more common.
I think people should hike or walk at least a couple times a week. It is one of those supper easy activities you can do on your lunch break, neighborhood, or gym.
You can also find parks in cities or travel outside. I have seen challenging hiking/walking trails in the city and way out in the middle of nowhere.
If you want to get in shape and enjoy nature I suggest you walk fast enough to raise your heart rate. Studies support that walkers are often in better shape than runners because of consistency (I think that is true for average people but not for fitness gurus.)