Sannyasa ashram can be taken up by person who has control over his senses and who longs for vairagya with undivided attention.
The following is the speech between Narada and Brahma in NARADA PARIVRAJAKA UPANISHAD OF ATHARVANAVEDA
That Brāhmaṇa who is a celibate, who has under control his tongue, sexual organ, stomach, and hand may become a sannyāsin without undergoing the ceremony of marriage. Having known samsāra as one without sāra (or essence) and not having undergone any marriage on account of the desire to know the sāra (or essence of God), they become sannyāsins on account of the practice of the supreme vairāgya. The characteristic of pravṛṭṭi (path) is the performance of karma; that of nivṛṭṭi is jñāna. Therefore placing jñāna in the forefront, the wise man should take up sannyāsa.
So if a person is good enough to take sannyasa ashram by having control over his senses then there is no need to look into age limits and based on this Adi Shankaracharya didn't break any rules.
The rules and age guidelines are for people who are born with senses flailing around and needs marriage to get them under control and desire vairagya.