I'm an adult, and have been for a while. Does that matter?
You have to be a child first. (this may have a few spoilers)
Once you manage to be a child again, get passed that gate when you're heading towards Hyrule Castle from the Market and you will see a split road. Take the one on the right and then you'll see a sign and a huge boulder. And yeah you obviously need bombs to blow the thing up.
Once you crawl inside play "Zelda's Lullaby" whilst standing on the Triforce design.
How do I become a child again?