According to Jewish tradition, what was Adam's level of physical maturity (apparent age) when he was created? Was he a baby or an adult?
... אמר רבי יוחנן: אדם וחוה כבני עשרים שנה נבראו ...
Rabbi Yohanan said Adam and Eve were created as 20 year olds
This is to be explained in accordance with the dictum of R. Joshua b. Levi; for R. Joshua b. Levi said: All creatures of the creation were brought into being with their full stature, their full capacities, and their full beauty, as it says, And the heaven and the earth were finished, and all the host of them [zeba'am]. Read not zeba'am, but zibyonam [their beauty].
Rab Judah further said: The bullock which Adam sacrificed had fully developed horns before it had hoofs, as it is said: ‘And it shall please the Lord better than a bullock that hath horns and hoofs’; the verse first says: ‘that hath horns’ and then ‘hoofs’. This supports R. Joshua b. Levi, who said: All the animals of the creation were created in their full-grown stature, with their consent, and according to the shape of their own choice, for it is written: And the heaven and the earth were finished, and all the host of them 7 read not zeba'am 8 but zibyonam. 9
@msh210 above brings this link
Actually, there are two opinions in the mishnah, whether Adam was created as a baby, or at an apparent age of 20.
I also (@msh210) did not yet find what he is referring to
As the author of the linked material... I was referring to R' Eliezer's position (despite it being in the gemara, not a mishnah, it is introduced with a "tanya"). The quote from RH 11a above is where R Yehoshua ben Levi is quoted as support to R' Yehoshua's opinion. The other side of that dispute is R' Eliezer, who says the world was created in Tishrei, with everything in its infancy. He doesn't mention Adam as an example, but he does consistently say everything was in its infancy. And R Yehoshua ben Levi is described as proof of R' Yehoshua over R' Eliezer.