I know that when it comes down to it, it's up to me and my body to decide if I'm too old to start martial arts but I've gotten very interested in putting my children into martial arts. I too would like to join but sore knees and an unpredictable back have me leary.
How old were all of you when you started? I'm turning 39 in october.
Part of it depends on why you want to join Martial arts. I am being 100% serious, but, although I bash it as a fighting form, if you want a low impact exercise, Tai Chi is very good. Most laugh because it looks kinda silly, but if all you want is something low impact and to stay healthy, it works.
Now, higher impact, good workout but also low fighting technique is Tae Kwon Do or some form of Kung Fu.
Now, if you want something high impact and good workout and good fighting technique, (I've heard) Krav Maga, Brazilian Jiu-Jitsu, Boxing, Judo, Sambo, or Muay Thai kickboxing. These are NOT to be taken lightly and you should stock up on Extra Strength Tylenol.
That's my 2 cents.