Vee - there is not going to be any weighting for age, though you can spin it how you want and say he might be the "5 years old and younger" champion(!). I have young kids and have been in a similar situation before. I brought my eldest to a comp at FlipOut (a London and SE meet) a couple of years back when he was 5 or 6. I said to him, just have fun and he really enjoyed playing the games with me and the experience. I basically said if you play all the games and try your best that's a job well done.For some reason, I'd thought someone had mentioned having a women and juniors section. I'd told the Flipper Beast that there would probably be some other children (aged 8 and 6) at the tournament, and he seemed happy with that until this morning, when he greeted me - out of bed - with a huge worry that he couldn't get a high score because the other children were bigger than him.
Hence, why I asked. He likes playing two- and three-player games with us on Godzilla, but he now gets quite disillusioned when he doesn't win.
Importantly there are 10 machines in the comp. He is extremely unlikely to come last on every single pin - he will probably beat a few people on at least a couple if not more than that due to the randomness of pinball (@thelab once beat people in a serious competition without even being present at the club - people plunged his balls for him and he still got more points than others trying their best!).
So I'd recommend telling him (as I do with my sons) just have fun, and if he beats ANYONE on any machine (he will) that is amazing as he is only 5 - and you can show him the standings for those tables. Don't worry about the overall standings. You don't even need to show him if you don't want to.
To finish my story about my son's first event - he had a great time, did well and it was nothing to worry about. He even beat Ben on 3 of 5 tables