Solution:

Dice (6 sided)

Explanation:
> I am always 1 to 6: numbers on dice are from 1 to 6.

> I am always 15 to 20: Sum of all the visible sides (1 top and 4 on the sides) always add up to 15 to 20.

> I am always 5: Five sides are visible.

> I am 21 when I am flying: When we throw the dice, we can see all 6 sides for a small moment which all add up to 21.

Solution:

A keyhole

Explanation:
It goes right through the door but is stuck to the door i.e never comes in or goes out.

Solution:

A kangaroo

Solution:

Future/Tomorrow.

Solution:

I am a needle

Solution:

Pawns in the game of chess.

Solution:

Eleanor Roosevelt

Explanation:
"L" in "or" = Eleanor
Rows of "elt" = Roosevelt

Solution:

Watermelon

Explanation:
Green outside.
A small white layer inside green.
Its pulp is red.
Lots of seed in that pulp.

Solution:

A Pack of cards.

Solution:

A bald Head

Explanation:
No one wants to be bald and if the person got bald then he doesn't want to lose his head.