I adopted Lady at 4.5 years old and it was a great age. Expect that he will need a potty training refresher as most do when they are rehomed. Just treat him like a puppy and take him out (or to his pads) on schedule and praise and treat when he goes. Don't leave him unsupervised and give him a chance to go in the wrong place or it will make it harder to train him.
I highly recommend this book. Many rescue groups give it to new adoptees. I got it when I adopted Lady and it was very helpful.
Amazon.com: Second-Hand Dog: How to Turn Yours...