Solution:

I am a Postbox

Remove as many letters as possible, still postbox remains the same.

Solution:

JASON

July(J)
August(A)
September(S)
October(O)
November(N)

Solution:

Double Letters

Solution:

A man, who crawls on all fours as a baby, walks on two legs as an adult, and walks with a cane in old age.

Of course morning, noon, and night are metaphors for the times in a man's (person's) life. Such metaphors are common in riddles. There were two Thebes, apparently this Thebes was the one in Greece. And this Sphinx was apparently not the one at Giza, in Egypt.

Solution:

Each word rhymes with its numeric position in the list. (e.g. 'gun' rhymes with 'one'
etc.)

Solution:

Bookkeeper (00 kk ee)

Solution:

Twinkle and Rinki start on opposite sides of the river

Solution:

1st person : Colin
2nd person : Rooney

They both lied.(I said at-least)
(If only one lied they would both be boys or both be girls)

Solution:

A lion, king of the jungle! (the head of a Lamb, the middle of a pIg, the hind of a buffalO and the tail of a dragoN)

Solution:

Velma waits until night time and then goes through the second door.

(No sun at night)