In case anyone subscribed to this thread, here are my amateur notes on the Kikuyu people and the ituika ceremony. This is a pretty blatant violation of the “answer” rules, but I figure I’m basically just talking to myself here anyways, so …

TL;DR ituika was being performed by the mid 1600s, probably.

Establishing a chronology of the Kikuyu people

In his introduction to A History of the Kikuyu Godfrey Muriuki notes the difficulty of mapping a people’s oral history to Gregorian calendar years:

Chronology is a basic requirement in any attempt to place historical events in perspective. Historical time has been called ‘the very plasma in which events are immersed, and the field within which they become intelligible’. Yet this is one of the main difficulties confronting a historian in Africa who utilizes oral traditions as a source of evidence. The problem arises because African societies did not measure time exactly. Consequently oral traditions or sources only give a vague picture of the absolute chronology of the events that they relate. In an attempt to overcome this problem, scholars have resorted to the use of lists of kings or rulers, generations and age differentiation systems, in order to establish a reasonable chronological framework. (Muriuki 14)

The Kikuyu, Muriuki writes, are “very historically minded” and use mariika – age sets – as “milestones of chronology” (15). This “institutionalized mariika system” varied by regional community but allows Muriuki to generate a cautious chronology of ruling generations.

Generations were distinguished by a two-category system of birth: every community member was categorized as mwangi or maina (also irungu). "If one generation is mwangi, their sons shall be called maina, and their grandsons be called mwangi, and so on" (Kenyatta 189)(see also Muriuki 117).

The origins of ituika/itwika

Jomo Kenyatta accounts a tribal legend of the rise of the Gikuyu system of government. According to the legend, the Gikuyu were at one time ruled by a despotic king. In response, an age set revolted (iregi, 'revolter').

After King Gikuyu was dethroned, the government of the country was at once changed from a despotism to a democracy which was in keeping with the wishes of the majority of the people. The revolution is known as itwika, derived from the word twika, which means to break away from and signified the breaking away from autocracy to democracy. This achievement was celebrated all over the country; feasting, dancing and singing went on with intervals for a period of six moons which preceded the new era of government by the people and for the people. [...] A revolutionary council, njama ya itwika, was formed to draft the constitution. (Kenyatta 187)

Ituika is the ceremony by which power is transferred from one age set (mariika) to the next:

It was decided that one generation should hold the office of government for a period of thirty to forty years, at the end of which the ceremony of itwika should take place to declare that the old generation had completed its terms of governing, and that the young generation was ready to take over the administration of the country. (Kenyatta 189)(see also Muriuki 117)

The generation after the iregi was called ndemi ('cutters,' "in remembrance of the period when the Gikuyu people began to cut down the forests and established themselves as agriculturalists").

I haven’t been able to meaningfully link Kenyatta’s account to any date ranges.^(1)

However, it is clear that ituika was a key faucet of the mariika system, and “we have clear evidence that the mariika system was a feature of the Kikuyu society by the middle of the seventeenth century” (Muriuki 116).

The ceremony itself

According to Kenyatta,^(2) the starting date of the ceremony was chosen by the elders.^(3)

The ceremony starts with a preliminary ritual and feasting "limited to the members of the council of the itwika." A public proclamation is then made of the new government, introduced through “war dances”:

[Community-wide war dances were] the only way through which the words, phrases and rhythmic movements of the new songs and dances, in which the laws and regulations of the new democratic government were embodied, could be introduced effectively into the life of the community. […]

[On the appointed day, a ceremonial leader] began to recite the constitution in the form of a song; the assembly answered in chorus after each phrase. This went on until the whole version was gone through. (Kenyatta 193)

“Ituika was a protracted affair said to have taken years to complete” (Muriuki 23); according to Kenyatta, the last complete ituika to have taken place lasted more than eight years, 1890-98 (189).

The ituika was also a particularly good time to clear and improve trade routes between Kikuyu communities (Muriuki 109).

When ituika was finished, the new government would rule for thirty to forty years before another ituika ushered in the next generation.

After the proclamation ceremony and feasts were finished, and the new government started to function, the revolutionary council, njama ya itwika, was dissolved, and delegates returned to their respective villages, leaving the officiating elder to take care of the ceremonial objects which were necessary for the perpetuation of the itwika. (Kenyatta 194)

Footnotes:

1. Muriuki’s chronology of Kikuyu ruling generations does include two mariika by the names of Iregi (1827-61±10) and Ndemi (1792-1826±15), but those do not obviously link to Kenyatta’s legendary account (an obvious problem is that in Kenyatta’s account, the ndemi held power after the iregi).

2. "Besides my general knowledge of the subject, I learnt the details of this ceremony from my late grandfather, Kongo wa Magana, who took an important part in the last itwika ceremony, performed between 1890-1898, and also from my relative-in-law, Kaambo wa Gethuku, who sounded the ceremonial horn during the same celebration in Ng'enda, a section of Kiambu district [a county north of Nairobi, Kenya]” (Kenyatta 189-90).

3. Elders of the two highest ‘age grades’ (also called riika, but distinct from the broader generational classification), the kiama kia mataathi and the kiama kia maturanguru (the eldest, 'dignified elders'), “decide the dates of circumcision feasts [initiation ceremonies] and the holding of the itwika ceremony when the older generation of rulers give place to another. Only the elders of these two grades can take part in the Gikuyu ceremonies at the sacred fig tree, mogumo” (Kenyatta 108-9).

Sources:

Kenyatta, Jomo. Facing Mount Kenya: The Tribal Life of the Gikuyu. London: Mercury Books, 1961. [First published 1938, Martin Secker & Warburg Ltd.] https://www.sahistory.org.za/sites/default/files/file%20uploads%20/jomo\_kenyatta\_facing\_mount\_kenya\_the\_tribal\_lifbook4me.org\_.pdf.

Muriuki, Godfrey. A History of the Kikuyu 1500-1900. Nairobi: Oxford University Press, 1974.