To live to 120 in Bitlife, you will need to maintain perfect stats on everything there is to your character.
This means you will need to have perfect health, happiness, and looks, and smarts. That said, you can raise happiness by doing activities that increase the stat such as watching tv, spending time with family and going out on activities with them or just in general.
Having a good job that pays a lot is also a bonus for happiness, as the money you earn can be spent on said activities and living a happy life.
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Activities under the mind and body will also help to increase happiness, as well. For smarts, you will need to read many books and go to the library; however, you can probably get by with just reading books because it can raise stats from zero to 100 with that alone.
To raise your looks, you will need to go to the gym or visit a plastic surgeon via the option under activities. However, going to one will cost a crapload of money, so you might be better off just going to the gym unless you have a decent job with a lot of money.
Now for health, you will need to have a good diet to keep it high, we recommend choosing the Mediterranean diet, as it can increase your happiness and health. Going to the doctors and avoiding being sick is also the best way to keep your health high and to increase it.
Check out some of our other awesome Bitlife Guides!