"no younger than I am" or "no more young than I am"?

In English, the idiom is: [subject] is no more [noun or adjective] than I am is very common. Please check out my examples:

He's no more a thief than I am.

He's no more rich than I am.

He's no more young than I am.

Those sentences above are idiomatically sound.

This idiom should not be confused with:

He's no richer than I am. He's no younger than I am.

In the first group: no more + adjective is not a comparative adjective. It is just a regular adjective. He is rich. He is not rich. He is no more rich than I am rich.

Please: "He's no more young than I am" implies we are both old. Just as: "He's no more rich that I am" implies we are both somewhat impecunious.

No more here means: His condition or state is not rich just as mine is not. Not at all rich. I'll leave poking at the grammar here to others. Frankly, I can't be bothered. :)

I am in complete agreement with @Lambie's answer. I thought this might be of interest:

Horn, Laurence R. and Wansing, Heinrich, "Negation", The Stanford Encyclopedia of Philosophy (Spring 2017 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/spr2017/entries/negation/.

1.7 Privation, affixal negation, and the markedness asymmetry For Aristotle, privation is an instance of opposition defined in terms of the absence or presence of a default property for a given subject: We say that that which is capable of some particular faculty or possession has suffered privation [sterêsis] when the faculty or possession in question is in no way present in that in which, and at the time in which, it should be naturally present. We do not call that toothless which has not teeth, or that blind which has not sight, but rather that which has not teeth or sight at the time when by nature it should. (Categories 12a28–33)

A newborn kitten, while lacking sight, is thus no more “blind” than is a chair, nor is a baby “toothless”.

From a message board http://forum.worldreference.com, on these sentences:

1 He is no younger than I am. (No one knows whether they are young or old.) 2 He is no more young than I am. (Both are old.)

2 is meta-linguistic: it's disputing the word 'young'. Young? Him? He's no more 'young' than I am! It can be used with noun phrases too: What, Jim an expert on chemistry? He's no more an expert than I am!

(Posted by entangledbank Senior Member, London, English - South-East England)

There are a number of examples of "no more rich than I am" in casual Internet use and a couple of instances of "no more rich than" in academic papers.

"No more young than" is less frequent and has the problem that "young" can refer to the offspring--no more kittens than last year.

By asking such a deep question you have unknowingly stirred up a grammatical hornets' nest! You can write "no more young" if you want to - the thing is that if you are expressing yourself creatively, especially in fiction, perfect grammatical correctness is not mandatory.

Unfortunately this seems mainly a grammar and usage forum; and phrases like 'no more young', 'no more rich' and 'no more cold' are grammatically unwelcome if not outright incorrect. No younger, no richer and no colder sound so much better!

But I understand what you are getting at, and you are right: "no more young" is quite different from 'no younger', and it can be appropriate in certain contexts, as in "Don't tell me she is young -- you are 62 and she is 60 -- young indeed! she is no more young than you are..."

By the by, if you go the 'less route' rather than the 'more route', wouldn't you have no choice but to say 'no less + adjective', as in "YOU SAY I AM TOO YOUNG FOR THE JOB? THEN YOU CAN FORGET JACK AS WELL: HE IS NO LESS YOUNG THAN I AM!" -- "It is no LESS COLD in Alaska than in Greenland" -- maybe you could have the cake and eat it too!