However, for most tournament play, you can not use the old cards. I believe that you can not currently use anything issued prior to EX Deoxys, but, please correct me if I'm wrong.
You also can not use any cards that were issued in Champion deck sets(they have a different border and generally have a signature on the card of the person who's deck the card originated from).
Sadly, as the cards are becoming more powerful, there isn't much sense to using the old cards. When the first sets were originally released, base set Charizard's HP of 120 was astronomically high. However, the EX cards changed that with cards carrying HPs of 150 and 160. The drawback to using them was that your opponent took two prize cards when they were defeated. However, with the advent of the Lv. X cards, you no longer have to worry about that additional effect, but the LV. X card acts as a 3rd evolution.