Is there any phrasal verb to mean "teeth are falling out because one is getting older"?

The medical term, edentulism, and its verb form, edentulating, might fit. But I suspect that they are not really suitable for conversations with anybody besides dentists, orthodontists, and lexicographers. The verb form, while not common, nevertheless appears to be in use.

I also noticed a couple of exotic instances of the word being used metaphorically:

But such logic hasn't stopped the court from edentulating itself before;

We often say someone is long in the tooth to indicate that they are old. It comes from the notion of the gums receding (prior, presumably to the teeth falling out).

I do not think such a word exists - it is a rather specific definition.

If a more euphemistic, figurative approach is acceptable, I would write:

"Age is taking his teeth."

It's simple, and to the point.

Also, it's "losing", not "loosing", unless he's making a conscious effort to "set free" his teeth.

As Robusto says, long in the tooth is a common expression. But more often than not, people aren't even particularly thinking of teeth when they say this - let alone the possibility that receding gums will probably eventually lead to tooth loss. It's just become a stock phrase meaning "aged, very old".

I can't find any relevant instances of this, but I think if you said...

"He's getting gummy in his old age"

...most people would understand what you meant.

I've belatedly taking note that OP isn't a native speaker, and has accepted the somewhat risible edentulating. That's a very "exotic" word, more likely to occur in facetious metaphorical contexts such as the one cited in the answer (an allusion to the law having no teeth). Like the single instance in Google Books, the handful of non-figurative instances on the Internet refer to dentists removing (usually, all) teeth for therapeutic reasons, rather than to teeth simply falling out due to old age.

Obviously I don't think He is edentulating now is suitable for OP's context. So I'm amending my answer to point out that his initial assumption is simply mistaken. As Reg comments, there's nothing "awful" or "dumb" about...

He is losing his teeth now because he is getting older.

A more "formal" (but still "normal") description might be...

He is undergoing [the natural process of] age-related tooth loss.