Hi, for the age/origin of the river, consider x-posting to /r/AskScience (Earth sciences category): the formation of the river is certainly from prehistory.

The Oxford English Dictionary lists the earliest reference to the Thames (spelled Temes) as occurring in 893 in an Old English/Anglo-Saxon document attributed to King Alfred. The Online Etymological Dictionary cites an example of the name in a Latin source - not given attribution - dating to 51 BCE. The word may be of Celtic/British origin (i.e. of Brythonic origin) meaning dark water, but its exact meaning is speculative. The early Latin source indicates that this was the name of the river when Roman forces first arrived. If it is of Celtic origin - which is not certain - that would place the naming of the river a few centuries before that time, but since that is before records, one cannot ascribe a specific date to the naming of the river.