Hi, Nick.  Language learning is largely a function of age and mental agility.  There are linguists who can hardly speak any language other than their native language, and there are non-linguists who have good command of several languages.  It won't hurt you to try.  I'm a year away from 70, but I've picked up a pretty good passive command of Italian over the past two years.  That is primarily because I've worked hard on vocabulary and traveled to Italy several times.  Knowing French and Spanish definitely helped, but they get in the way of Italian.     However, my ability to retain vocabulary, especially for active use, degraded steadily as I grew older. 

In my experience, the key to learning a new language is primarily in building up your vocabulary, because that gives you the ability to understand more of the language, even if your conversational skills are lacking.  If you keep at it, your progress will be steady, even at an advanced age.  Here's the thing, though.  Arabic is sufficiently different from the other languages that you know that your progress may be slower than you wish.  You won't have a lot of vocabulary from related languages to interfere or help.  So be patient and keep at it.