Imam Shafiee rahimahullah said:

أخبرنا خيثمة بن سليمان بن حيدرة القرشي قال: سمعت أبا إسماعيل الترمذي يقول: سمعت البويطي يقول: سئل الشافعي: كم سنك، أو مولدك؟ فقال: ليس من المروءة أن يخبر الرجل بسنه

“al-Booti said: I asked Shaf’iee rahimahullah: How old are you or when were you born? He said: It is not from the manhood for a man to inform [others] of his age”

[Reported by Qaadi Abul-Hasan al-Halabi and the chain is Saheeh]

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan

———-

So it is only a recommendation. As Imam Malik rahimahahullah said:

عبد العزيز الأُوَيْسِيَّ قال: قال رجل لمالك: با أبا عبد الله، كم سنك؟ قال: أقبل على شأنك

Abdul Aziz al-Uwaisi said: a man said to Malik: Oh Aba Abdillah, how old are you? He said: Turn towards your own affair!”

[Reported by Qaadi Abul-Hasan al-Halabi in al-Fawaaid and the chain is Saheeh]

However, it is allowed to inform people of his age and it is not prohibited. Ibn al-Arabi rahimahullah said:

فلا بأس أن يخبر الإنسان بسنه ، كان صغيرا أو كبيرا .

“There is no harm in a person informing a person of his age, whether he is young or old”

[Ahkam ul-Quran]

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan

Ibn Jawzi رحمه الله mentions in his book Sayd al-Khaatir some of the reasons for such a suggestion.

Ibn Jawzi , rahimahullah, says:

فإنه إن كشف مقدار سِـنِّه استهرموه ( رأوه هرماً ) إن كان كبيرا، واحتقروه إن كان صغيرا ،

“For verily, if he were to reveal his age while he is old, then they would see him as one who has become senile. And if he is young, they would look down at him (for his age).” End translation.

Translated by

AbdulFattaah Bin Uthman
Abu Fajr