How can I make my summoned mobs despawn?

For a 5 second lifetime, you need to use the command:

summon creeper ~ ~ ~ {Age:100}

You can always use scoreboards,

First type in chat: “/scoreboard objectives add Death air”

Add an always active repeating commandblock with “/scoreboard players add @e[tag=“Death”] Age 1”

Then add another always active repeating command block with “/kill @e[scores={Age=20}]”

(Note 20 ticks is one second, and since the repeating commandblock is basically reactivating every tick, each mob will be given their own 1 point every tick, and once they reach 20, that means they’ve been existing for one second.

You’re ofc free to change 20 to any amount of seconds such as if you want 6, you just multiply 6 • 20. Also note that you have to summon mobs with tags aswell as unlike despawning, the items drop.