Most all of my people live to the age of 65+ years, with the average being 67. (I've digressed lately since I added a few more families.)
I don't think it's so much the amount of food as it is the type of food when you are trying for age, as long as they have enough to last the length of time you have the game closed. They need organics in order to get those last couple of years in. We've had a recent discussion about groceries
in this thread that may be helpful in that area, but basically just try to clean out your fridge out using one of the events and start them over with only organics.
Don't leave them alone for long periods or an illness might creep in and take away that hard earned health. Once they are into their sixties, I check on mine more often to make sure I catch any illness early so their health doesn't suffer. Basically, only leave them alone overnight to make sure they get enough sleep. With your version, you also need to make sure they learn to nap so that energy isn't a problem.
(Assuming you have the bathroom, and air purifier upgrades) training them to exercise is great and is exactly what you want to do. Number of kids has no bearing and kids in the house also doesn't matter, other than having someone around to help out on chores, cooking, etc. It won't have any relation to how long they live though.
Keep some fruit, antibacterial hand gel, vitamins and maybe an energy drink on hand in case they get sick (the energy drink is in case you need to use antibiotics during the day.) When duplicates are available in the store, use the ones you have on hand to boost health and praise exercise, then replace your stock.
If you can check on them often enough to provide three meals a day, do so. If it's only twice, then that's ok, especially with the version of the game you have, because (if I remember correctly, their fed bar doesn't drop very quickly anyway, but groceries sure do.)
You CAN get the 70+ with the version you have. I had a few before I upgraded, but I have to tell you, it's much, much easier with the updated version of the game. The treadmill will help you a ton. It seems to slow the rate at which their energy and fed bar decreases, which is great since you don't have the fixes for energy consumption or the changes to food consumption yet.