Both of our boys took their first plane rides when they were about 3 months old. We took our older son to Puerto Rico when he was 6 months old - and we had a wonderful time! Although air travel is not the luxurious treat it once was, it is still manageable with kids of any age.
In a backpack (when they’re older, kids can pack and carry their own), bring some of your kids' favorite things. Include a special blanket or stuffed animal. Pack small, lightweight toys that are old favorites, as well as a few new (or borrowed) surprises for times when they're bored.
Some things that work well for younger ages when traveling are:
paperback picture books
crayons or colored pencils (in a Zip-loc bag) with paper or coloring book
simple age-appropriate "workbooks" or activity books
lacing or sewing cards or blocks
a gallon-sized Ziploc bag filled with Duplos or other toddler blocks.
Consider what your kids enjoy playing with most when they're at home. When our boys were younger, they loved playing with dinosaurs, animals, and action figures. When we traveled, we brought along some small-sized figures (another Zip-loc bag!) for them to play with.
Older kids can pack their own favorite pastimes:
Handheld electronic games (with earplug or sound turned off)
iPod loaded with music and movies
Deck of playing cards or other small games
Kids of any age can also listen to audio books on a portable CD player or iPod.
Keep Kids Fed
A hungry kid is a cranky kid. In these days of ever-decreasing service on airlines, you should bring along your own snacks, though most kids get a kick out of the beverage service. We often bring granola bars, apples, or nuts. For long flights, we bring along energy bars like Clif bars which are more filling.
There are now strict limits on bringing liquids or gels through security, with exceptions for baby food, formula, and medications. Check the TSA website before you fly for the latest update on restrictions.