Hence a young man is not a proper hearer of lectures on political science; for he is inexperienced in the actions that occur in life, but its discussions start from these and are about these; and, further, since he tends to follow his passions, his study will be vain and unprofitable, because the end aimed at is not knowledge but action.

[διὸ τῆς πολιτικῆς οὐκ ἔστιν οἰκεῖος ἀκροατὴς ὁ νέος: ἄπειρος γὰρ τῶν κατὰ τὸν βίον πράξεων, οἱ λόγοι δ᾽ ἐκ τούτων καὶ περὶ τούτων: ἔτι δὲ τοῖς πάθεσιν ἀκολουθητικὸς ὢν ματαίως ἀκούσεται καὶ ἀνωφελῶς, ἐπειδὴ τὸ τέλος ἐστὶν οὐ γνῶσις ἀλλὰ πρᾶξις.]

Aristotle (384-322 BC) Greek philosopher

Nicomachean Ethics [Ἠθικὰ Νικομάχεια], Book 1, ch. 3 (1.3.5-6) / 1095a.2-5 (c. 325 BC) [tr. Ross (1908)]
    (Source)

(Source (Greek)). Alternate translations:

Hence the young man is not a fit student of Moral Philosophy, for he has no experience in the actions of life, while all that is said presupposes and is concerned with these: and in the next place, since he is apt to follow the impulses of his passions, he will hear as though he heard not, and to no profit, the end in view being practice and not mere knowledge.
[tr. Chase (1847), ch. 1]

And hence it is that a young man is not a fit student of the art political; for he has had no experience in matters of daily life, with which matters our premises are concerned, and of which our conclusions treat. And since, moreover, he is prone to follow his desires, he will listen without purpose, and so without benefit. For the true object of ethical study is not merely the knowledge of what is good, but the application of that knowledge.
[tr. Williams (1869), sec. 3]

Hence the young are not proper students of political science, as they have no experience of the actions of life which form the premises and subjects of the reasonings. Also it may be added that from their tendency to follow their emotions they will not study the subject to any purpose or profit, as its end is not knowledge but action.
[tr. Welldon (1892), ch. 1]

And hence a young man is not qualified to be a student of Politics; for he lacks experience of the affairs of life, which form the data and the subject-matter of Politics. Further, since he is apt to be swayed by his feelings, he will derive no benefit from a study whose aim is not speculative but practical.
[tr. Peters (1893)]

Hence the young are not fit to be students of Political Science. For they have no experience of life and conduct, and it is these that supply the premisses and subject matter of this branch of philosophy. And moreover they are led by their feelings; so that they will study the subject to no purpose or advantage, since the end of this science is not knowledge but action.
[tr. Rackham (1934)]

That is why a young person is not a suitable audience for politics. For he has no experience with the actions of life, and the accounts are in accord with these and concerned with these. Further, since he tends to follow his feelings, it will be pointless and not beneficial to him to be in the audience, since the end is not knowledge but action.
[tr. Reeve (1948)]

In view of this, a young man is not a proper student of [lectures on] politics; for he is inexperienced in actions concerning human life, and discussions proceed from [premisses concerning those actions] and deal with [those actions]. Moreover, being disposed to follow his passions, he will listen in vain and without benefit, since the end of such discussions is not knowledge but actions.
[tr. Apostle (1975), ch. 1]

This is why a young man is not a fit person to attend lectures on political science, because he is not versed in the practical business of life from which politics draws its premisses and subject matter. Besides, he tends to follow his feelings, with the result that he will make no headway and derive no benefit from his course, since the object of it is not knowledge but action.
[tr. Thomson/Tredennick (1976)]

This is why a youth is not a suitable student of political science; for he lacks experience of the actions of life which political science argues from and about. Moreover, since he tends to be guided by his feelings, his study will be futile and useless; for its end is action, not knowledge.
[tr. Irwin/Fine (1995)]

This is why a young person is not fitted to hear lectures on political science, since our discussions begin from and concern the actions of life, and of these he has no experience. Again, because of his tendency to follow his feelings, his studies will be useless and to no purpose, since the end of the study is not knowledge but action.
[tr. Crisp (2000)]

Hence of the political art, a young person is not an appropriate student, for he is inexperienced in the actions pertaining to life, and the arguments are based on these actions and concern them. Further, because he is disposed to follow the passions, he will listen pointlessly and unprofitably, since the end involved is not knowledge but action.
[tr. Bartlett/Collins (2011)]

Note that this passage was the basis for these lines from Shakespeare, Troilus and Cressida, Act 2, sc. 2, l. 165 (1609):

Young men, whom Aristotle thought
Unfit to hear moral philosophy