1. You are not 50+ yet... "Half century" sounds kind of.... But You still have 10 years to create your list why being 50+ is a good idea too. Japanese may help. for them 50+ is age of blossoming and enjoyment.
2. You may become president. In different countries different age census, but in general, 40 years is considered solid age for you to lead your country. GO!
3. You have still 10-20 years to pay your mortgage. Your bank really cares about you. Every month. They may offer you discount on health insurance.
4. Most of your friends are still around. Just keep remembering about them. And by know, you have realized value of real friend.
5. You're old enough to be taken seriously. Remember back in your 20s when you tried to make a point, and folks would just nod and smile? Now you open your mouth, and people listen.
6. But, you may have also learned to not to take yourself too seriously. Read more here.
7. By now, you may know a thing or 2. If you have not been hiding in cave (and even that might teach you a lot) last 40 years, you have learnt something. You know something. That is cool. You feel wiser than those boys and girls 39-...
8. You have figured out by know people thare really matter for you -- and you may know that it is a surprisingly small number.
9. You know when to say no. Gone are the days of saying yes because you're afraid of hurting someone's feelings. In your 40s, you've figured out that the word "no" isn't inherently an insult.
10. You still CAN! That is great, to realize you can. Still. Anything. Almost anything. Learn new language. Ikebana courses. Karate, box, actor courses, change of career, are still all open options.