Reish Lokish suggests that one looks 'into the future' that when this animal grows older these cartilages or sinews will become hard....
Surely one should look 'be'asher hu shom', otherwise use the same the logic and suggest you can't use a lamb (e.g.) of 7 months old as it will become older (than a year) and eligible to be used
Sholem, UK
Sholom Rav.
An interesting question. In answer, let me work backwards. A sheep in its first year is called a lamb ('Seh'), which is what the Torah requires as a Korban Pesach. Consequently, the fact that a lamb will become a sheep is totally irrelevant.
In contrast, the Torah specifies Basar that has to be eaten of the Pesach ("ve'Achlu es ha'Basar ba'Laylah ha'Zeh"), and the definition of 'Basar' is not always clear-cut - such as in our case, where the cartilages and sinews will become inedible at a late stage. Hence it is perfectly understandable that Resh Lakish (and ultimately also R. Yochanan who retracted) maintains that since they are going to become hard at a later stage, they do not fall under the category of Basar.
Admittedly, I am not clear why they gauge it by the cartilages and sinews of an ox rather than of a sheep, but that's another question.
be'Virchas Kol Tuv.
Eliezer Chrysler.