Udena, the king of the Vaṅsas, questions the venerable Bharadvaja as to why respectable young men of family would renounce the world and live their entire lives as beggars in the Dhamma taught by Gotama. Bharadvaja provides him with several answers the last of which satisfies the king who then becomes a lay follower.
Read the Sutta
Index of Available translations: SN 4.35.127
A point of interest here is the translation of the first response where Bharadvaja quotes the Buddha as teaching that the bhikkhu should mātu-mattīsu mātu-cittaṃ upaṭṭhapetha 'for the mother-measured mother-heart set up'.
Woodward: "In the case of those who are just mothers, sisters and daughters, do ye call up the mother-mind, the sister-mind, the daughter-mind."
Towards those considered (measured as, reckoned to be; Woodward's 'just' = 'a measure of') mothers, set up the heart as towards a mother, etc.
Per Bhk. Bodhi: "towards women old enough to be your mother set up the idea that they are your mother."
Bhk. Thanissaro: "with regard to women who are old enough to be your mother, establish the attitude you would have toward your mother."
The idea 'old enough,' presumably the translation for 'measured as,' is pushing a narrow idea onto the Pali. It's 'measuring'. Taking into consideration all impressions together there is a similarity to ... .
Think of the custom in countries like China and India where those towards whom one has feelings as towards a mother, father, uncle, brother, sister, daughter, are so called even between strangers.
The idea here is 'who are similar to'.
Age will be one factor of many.
The idea that the mental attitude is to be the one one has for one's own mother, etc. is also not indicated but is being picked up from commentary. That could be a dangerous proposition! (as is pointed out by the king in response). Both sons, brothers and fathers and mothers, sisters and daughters are subject to deviant thoughts and feelings about each other.
The idea is to set up the ideal way one should consider mothers, etc.
This sutta is being delivered by the bhikkhu named by the Buddha as one who is best at uttering a 'lion's roar'. In the case of this bhikkhu his lion's roar is that he is able to answer the questions of anyone concerning the paths and benefits from Streamwinner to Arahant. This sutta however does not seem to show him at such a high level as his first two answers are both incomplete responses to the question and ineffective in satisfying the King.