If you search the town for seeds, some are labeled "special." These can be four or five different things, one of which is a Life Plant. You can't figure out which until after it has fully grown. Anyway, if you eat the Life Fruit that you harvest from the plant, it will decrease your age by 1 day. This requires a high (level 10?) gardening skill.
If you have level 10 cooking, gardening, and fishing, you can catch a deathfish and harvest a life fruit, buy the 12,000 simolean recipe and make ambrosia, which resets the age bar for that sim (won't turn back from elder to adult or teen to child or anything like that), and give a +75 moodlet for 7 days.
Or, you could just go into the settings and turn aging off temporarily, or increase the lifespan slider.
you probably wont be able to achieve it before you die tho, it can take a long time. but you can have other member of the family to bring you back from dead in a ghostly form at the science lab (which you need to wait for an "opportunity") then once you back, you can play your sim as regular sim and keep working on the gardening/fishing/cooking/finding seeds etc then once you get ambrosia you can restored back to a regular sim