It's totally fine to be forty as long as you have a grasp of how to play the game.
The format is as follows:
1) registration, check in, pay, give your Play Pokemon number (you can usually register on the day)
2) deck building, open your 23 card set and 4 booster packs, spend time building a 2/3 (40 card) deck with the cards you've received. The organisers will provide energy cards, but everything else has to come from the cards in your build and battle kit
3) tournament against others with the deck you just built. As it's 2/3 of a full deck you play 4 prize cards
4) prize giving
I would recommend taking with you:
- 7 dice - used instead of coin flip evens head odds tails and to count damage taken
- card sleeves as you'll be building the deck on the day, you'll end up playing the "expensive" cards you pull. I like Dragon Shield.
At the last one we went to (Southampton) all prizes were given in advance and equally, so three extra boosters per player - I think this is because there are a lot of kids. Then there were enough players to need 5 best of 3 rounds to find the winner (me!). We arrived about 11:00 and were done by 17:00. Good value for money I think.