If you want to check your brain age and you're not good at all of them, you can select the tests you want to do. Just click select on your DS and at the same time click THE BRAIN AGE thing. It should take you to a screen.
The easiest(in my opinion)is Highest Number, Speedy Symbols & Memorise 5X5 (I just write it down on a piece of paper and then copy!)
Now I know this isn't from memory but it's a good way to get a young brain age and not get caught by the weird talking head on the DS screen just get a piece of paper and a pen.Then while the two minute countdown is going for you to memorise the 25 numbers just write down the numbers in their exact places on the piece of paper. When you've finished and checked they're correct press select and you just write them down in the squares. At this point you just have to remember to write the number clearly. Hope this was helpful!! Lol