I know that astronomers have detected supermassive black holes in the early universe. What was the age of the universe at which these black holes were detected? In other words how long is it thought to take for these supermassive black holes to form and what are the mechanisms by which they formed, other than from supermassive stars?
The first supermassive black holes must have been formed earlier than about 750 million years after the big bang, since there is evidence for luminous quasars at redshifts of up to 7 and with likely black holes of a billion solar masses or more at their centres (e.g. Momjian et al. (2013).
The candidate mechanisms to produce such objects in a short period of time are hyper-Eddington accretion onto stellar black hole seeds; the direct collapse of large, primordial gas clouds into black holes; or the merger of stellar-sized black holes in dense clusters, followed by gas accretion. These possibilities are accessibly reviewed by Smith, Bromm & Loeb (2017).