Item summoned with that command :
/summon Item ~ ~-5 ~ {Item:

,Age:15}

didn't despawn after 15 games tics, and it doesn't work for any Age and any item.

Steps :
• Take a command_block
• Write inside [ /summon Item ~5 ~ ~ {Item:

,Age:20}
• What should happen : It should despawn after one second.
• What happened for me and may happen for you : It won't despawn until 5 mins.

I hope i did everything good and this is a real bug, and sorry if i made you loosing your time (and sorry for the grammar i am french )