If I were to make an asset for Unity, and would want to sell it on the Unity Asset Store, would I need to be 18 or older?
Seeing as you have to be 18 for PayPal, I’d suggest not putting something up until you’re 18.
What if I already have a Paypal account? Parent has cosigned 
I did the same, but I thought it was safest to check 
Second quick question:
Do I have to handle my own taxes? Like, if I have a home-based business, you have to report your own taxes to the US Government. Is it the same for Unity?
You have to pay estimated taxes quarterly, regardless of the source of income. Yearly, you have to file your actual taxes, and either pay the difference, or get a refund. If you have underpaid by more than 1000 dollars, you will also pay a fine.