Yes, you can. The church welcomes new members however old they are when they convert.
The normal way to convert would be to speak to a priest, and tell him that you are interested in becoming Catholic. You would then go through a period of a few months when you recieved instruction in the Catholic faith, to make sure you understood things enough to make an informed commitment. Then, if you wanted to proceed, you would be formally recieved into the church, and would be able to start recieving the sacraments.
If you have already been baptised as a Protestant, the Catholic church would recognise that as a valid baptism. So there would be no need for you to be re-baptised when you ecome Catholic.