Is it legal for a 13 year old to charge for dog-walking?

Im 13 and i want to start dog-walking (maybe sitting, too), but im curious to know if i can get sued or something for charging while under-aged and unlicensed. Is is legal for me to do this at all? And what should i know about it? i already have an idea for a logo and prices, but is there anything else i should know and watch out for? Also, should i make the owner sign something, like saying that thier dogs are vaccinated or as like a release voucher? Or like saying that if i get hurt i can sue them or something like that?