As far as reptiles are concerned there are very few rules about keeping a native species as long as it is not a protected species or considered invasive, for instance in CA common snapping turtles are against the law to possess because they don't want them to escape and become established in the state. I don't know anything specific about PA but I don't remember any restrictions to sending mud and musk turtles to PA, I know there are some rules for selling native reptiles, for instance most have to be captive bred but you can collect the breeders from the wild as long as you are not selling the ones you are collecting. There are exceptions, again I don't know the specifics of every state but in CA you are allowed to collect and sell certain box turtles with the correct permits even though they are native and as a hobbyists you can collect 2 for your own personal collection with nothing more than a fishing license. Even in the states that require a fishing license to collect your own I would think for a person to find one in his or her yard there is probably no reason anyone would check, although technically you may be correct if a license of any sort is required. There is a great book, can't think of the author but the title is Reptiles and the Law, it covers all 50 states and the federal laws, and I don't remember any restrictions on any species of mud or musk turtles if they are legally purchased or collected, but I am not an expert on individual state laws. I don't know where this individual was found but as I mentioned as long as you are not going to try to sell it there should be no reason you did anything wrong.
Keeping a baby turtle you find in the wild should be no different than purchasing one, they don't have any parental bonds or need siblings or companionship, they are very easily kept if you have the correct set up so in my opinion (and I don't hold it against anyone who disagrees with me, I have friends who don't think any animal should be kept in captivity which is the opposite of my line of work, I own an exotic animal breeding and quarantine business so I have dealt with wild animals most of my life) if you are willing to care for it more power to you. What I will say is if you are not going to give it the set up and care it needs to thrive, find a suitable place and let it go or give it to another hobbyists who is willing and able to care for it.