Question about when kickers apply in Hold'em

[Thirdman]

Hi,

I have a question about when kickers apply, I have just been in a game:

Me:

Ac Td

Board:

7d Ah Qh 7c 5h

Opponent:

Ad Jd

The result was a split pot, each player having 2 pair A and 7. This blew me because I thought that my opponent would have won with the J kicker.

An explanation would be much appreciated thanks.