I'm reading the Wikipedia page of Upanayana,
Many medieval era texts discuss Upanayana in the context of three of the four varnas (caste, class) — Brahmins, Kshatriyas and Vaishyas.The ceremony was typically performed at age eight among the Brahmins, at age 11 among the Kshatriyas, and age 12 among Vaishyas. Apastamba Gryha Sutra, in verse 220.127.116.11, places a maximum age limit of 24 for the Upanayana ceremony and start of formal education. However, Gautama Gryha Sutra and other ancient texts state that there is no age restriction and anyone of any age can undertake Upanayanam when they feel they initiate their formal studies of the Vedas.
Several texts such as Sushruta Sutrasthana, however, also include the fourth varna, the Sudras, entering schools and the formal education process, stating that the Upanayana samskara was open to everyone.
As already said, some texts say Shudra should do Upanayana as well but what age should they?