So, in addition to him not needing to eat, drink, sleep, breath, etc. does the Crimson Gem of Cyttorak also give Juggernaut eternal life? I don't know if it was ever stated whether Juggernaut never ages or if he just ages extremely slowly, or does Juggernaut age at the rate of a normal human?

While under the power of the Gem he does not age. Normally. In the realm of oblivion it aged and de-aged him trying to kill him. But it couldn't.

Since characters rarely age in the Marvel Universe... does it matter? ;)

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Since characters rarely age in the Marvel Universe... does it matter? ;)

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No, thanks to Cyttorak making Juggy Immortal he is unable to age physically and internally. 
 
But still has an age

Cain told Xavier he was older then him by all the time he spent in other dimensions & such but he never ages just stays the same age of a man in his late 20's early 30's  .

Juggernaut doesn't age, he is a timeless classic. Put him in your DVD player, and relive classic and hilarious highlights, over and over without pause. Call now, and you'll get a Franklin Richards, who not only doesn't age, but reverses the aging process. Massage him into wrinkles to watch them magically disappear. Give him to your wife or girlfriend. Studies and science polls show the thing they fear most is becoming old and infirm. Well Franklin Richards cures that. Just apply a glob of him daily on skin, to reverse the effects of aging.  
 
**Doctors general Health Warning, though Juggernaut is not a mutant, Franklin Richards is, and so like all mutant products, there is risk of hair loss, memory loss, vomiting and extra increased risks of turning into a mutant yourself.**