Those with high level Life Experience...the Elderly:
I'm far from 'elderly' but as a mid-forties mom, I've been around the block. I want to share my lessons with my boys to help them avoid repeating my mistakes or wasting time. Of course, sometimes they don't want to listen, and sometimes I don't understand the things they are talking about. (Like anything Xbox, technical, etc.) This is, in essense, where the elderly typically get lumped/typecast in a story. But the non-twenty-and-thirty-somethings have much to offer.
Even Bilbo Bagginns at 111 was pretty kick ass, though that was because of the ring. Wizards are typically older, but still very vital, if Gandalf is any indication--especially his fight with Saruman. Han Solo and Leia and Luke are passing of the torch with heartbreaking grace. Personally, I loved that scene in Battleship where the old vets run the museum ship and attack the enemy. I thought that was fantastic, showing their input remains vital...but then as a former VFW bartender, vets are special to me.
Outside of those charaters, when I think of elderly characters, I think of Dragonslayer.
Ralph Richardson played Ulrich, the wizard. Spoiler: after giving cryptic instructions to his apprentice, he allowed himself to be killed. The instructions were for his resurrection. Because he was too old to make the journey. And because wizards are sneaky shits.
If you're worldbuilding, consider what hardships and benefits the older generation might have. How can you use this or show this in your story? Does a supporting character have a grandpa they have to help as well as the hero? If the heroes gramma is a major motivator, does the hero/ine see that gramma is focused on them instead of their own woes, and if so what impact does this have on hero/ine? What gut emotional impact can you bring to your story by allowing a glimpse or hard scrutiny of the elderly around the hero/ine?
Those with minimal Life Experience...the Kiddoes:
The fun here is watching them discover something for the first time, or seeing them putting it all together in their head and having their own opinions and ideas about it, whatever 'it' is.
The Harry Potter series covered a great scope of childhood, from magic candy, magic sports, and magic school pressures. Rowling hit on family issues good and bad, peer pressure, clothes, grades, everything. Stranger Things has zeroed in on some pretty intelligent kids (D&D gamers, so creative but truly thinking) from the 80s and I dig their story and characterization.
But those are both modern-era. What about kids in medieval-type times? Oh, yeah. George RR Martin has shown the perks of bloodline, deaths for being in the wrong place at the wrong time, the surprise of being sacrificed, and the upside of being dangerous and 'overlooked.' As for futuristic, space? I dunno. Does Wesley Crusher count?
In your world, how are children viewed? Are they educated, well fed, and do they have toys? Are they spoiled, lazy and distracted? Or are they taught to revere information? Are they given choices when they are understandably too young to understand the consequences? They are typically all about self-discovery. How can you step aside from that in your world and bring something fresh to the page?