Its usually hard to gauge the exact age of a Chameleon. But going off the pictures, i want to say, yours could be around the 5-7 months old. If you bought your Chams from a breeder vs. Petco/Petsmart, you can always ask them the age. They keep proper records
Many great jackson's caresheets on the forum. but to just answer your questions...
Feeding - Variety is key. I feedback my Jacksons Crickets, Dubia Roaches, Discoid Roaches, Silkworms, and Hornworms. Waxworms on occasion as treats (High in fat content).
I have been advised to avoid mealworms. Although they are easily available, they apparently aren't great for a Chams digestive system.
I am lucky in the sense that I have a specialty reptile store in town that carries all the feeders, so i just make 1 trip every 2 weeks and pick up feeders. See if you have a store like that close to home
Life Expectancy - Again, not an exact figure but a well cared for Chameleon can live for anything between 4-7 years. At 1 year old, they are referred to as mature.
Here are some of the links I bookmarked when i first got my Chameleon
https://www.chameleonforums.com/blogs/hoj/634-montane-chameleon-care-info.html
www.chameleonforums.com/jackson-chameleon-279/
https://www.chameleonforums.com/montane-care-sheet-critique-plz-79628/
https://www.chameleonforums.com/keeping-jackson-s-outside-86324/
https://www.chameleonforums.com/frequently-asked-questions-71620/#post668825
https://www.chameleonforums.com/silkworms-101-a-7906/
One piece of advise i would give is to find a local vet that specializes in Chams
http://www.arav.org/find-a-vet-temporary/
A quick check up of new Chams doesn't hurt. But in incase of a medical emergency its good to have a vet that knows your Chams history
Finally, are both your Chameleons in the same cage?? If so, you might want to separate them. Chameleons are solitary and territorial. The reason your second one (Female?) is black because is she stressed out by the other one. Stress can kill a Chameleon pretty quick. My advice, either get a second setup going ASAP, or return one form where you bought
Good Luck!