Never give your age online, it's not safe - so, without knowing exactly....
When I was 10, I'd walk dogs to exercise them for their owners. There's actually decent money in that, and my parents knew I was safer walking dogs then I would have been walking by myself. I also did recycling.
At 12, I made extra money doing yardwork and I started babysitting around that age.
By 14, I didn't just walk dogs, I pet sat as well. I'd bring in mail, water plants, feed fish, clean the litter box - there's a good bit of call for that, folks can board dogs but someone has to come to your house to feed your parrot.
By 16, I also cleaned houses and even did a bit of handy-work. If you can really neatly paint a bathroom or closet, or pull hair out of a drain, most folks don't want to call in a pro for such small jobs, but are happy to pay you if you're already working for them some other way anyway.
~but about that, if someone hires you to walk the dog, and then asks you to do ANYTHING extra - be sure to charge! If they say "Oh, on your way past, pick up the mail", or "While you're in the backyard, why don't you scoop the poop as well" Always say "Sure, I charge $X for that service!" right off the bat. Otherwise you will find that someone is paying you to do one thing, and you spend an extra hour there doing all manner of just little bitty things and you're not getting paid (and later in life, you'll find most employers will try to take advantage the same way, so this is good practice)
But be sure to have a parent check out every client!! Once there was this creepy guy who asked some really inappropriate questions - I had my Dad screen all my clients after that. Always be careful and stay safe.