I have a good and bad experience with renting a room, thankfully only 1 bad person out of 3 and he was turfed out after a couple of days after breaking the house rules.
You need to have very specific house rules laid out in terms of the following, smoking, cleaning, noise. Once everybody knows what is expected of them, and both parties have a clear discussion agreeing to the terms only then should you agree to rent out a room to them. But of course rules do get broken so you need to be firm and ask them to leave if what they are doing does not fit in with your home, eg smoking.
For an older person I think a younger student can provide a lot of company, and both can learn from each other, kinda like having a bonus grandparent if you get lucky!
I know for us the money is too good to say no too, means we can have holidays, there would be none without it and all work and no play is no way to live.