Found an old gameboy (10+ years) that the batteries had exploded in. is old battery acid still extremely dangerous if it is on my skin? washed hands.

You're ok: The danger from exposure to battery acid is an immediate chemical burn. If you didn't burn yourself and you have already washed your hands with a lot of water then you're ok. Soap or vinegar are basic and neutralize the acid but the important thing is to wash as quickly as possible with a lot of water. Just don't touch it again. And don't pour water on it, you can spread the acid. Scrape it off.