The answer is... yes! Hehe.
When anyone eats Ambrosia, their age bar resets to the beginning of their current age range. So, if an adult who is due to age up to elderly in a couple of days eats Ambrosia, he/she will be right back at the beginning of the adult stage. Everyone else around them will continue to age normally, even their kids, unless they also receive some sort of life extension. It's very bizarre sometimes when that happens, but it's fun when you're attached to your Sims. I've got a family now who eats it regularly because I can't bear for them to die just yet. Three of them are elderly, but one is a young adult. She eats the Ambrosia that her mother makes.
You're right that you'd need someone with the highest levels of fishing, gardening, and cooking. But it doesn't have to all be in one Sim. If you have a fisherman who can catch Deathfish, a gardener who can grow Life Fruit, and a cook who knows the Ambrosia recipe, you're set. Just put the ingredients into the cook's inventory (or into the refrigerator) and it's good. Hang in there past level 6 and you'll be having a good time, for sure!
