"And the rams of your flock I have not eaten" (Genesis 31:38). Rava said: an ox a day old is called an ox, as it is written (Leviticus 22:27) "an ox or a sheep or a goat, when it is born." A ram a day old is called a ram, as it says, "and the rams of your flock I have not eaten." It was rams that he did not eat -- but lambs he did eat? Rather, learn from this that a ram a day old is called a ram.
A Day-Old Ram Is Already Called a Ram
Yalkut Shimoni on Torah 130:14
וְאֵילֵי צֹאנְךָ לֹא אָכָלְתִּי. אָמַר רָבָא, שׁוֹר בֶּן יוֹמוֹ קָרוּי שׁוֹר, דִּכְתִיב (ויקרא כב, כז) "שׁוֹר אוֹ כֶשֶׂב אוֹ עֵז כִּי יִוָּלֵד". אַיִל בֶּן יוֹמוֹ קָרוּי אַיִל, שֶׁנֶּאֱמַר וְאֵילֵי צֹאנְךָ לֹא אָכָלְתִּי, אֵילִים הוּא דְּלָא אָכַל הָא כְּבָשִׂים אָכַל אֶלָּא שְׁמַע מִינֵּיהּ אַיִל (בֵּין) [בֶּן] יוֹמוֹ קָרוּי אַיִל.