Training of Jedi younglings ranged between the ages of 4 and 8 but at what age was a Jedi able to feel a Force sensitive child? Is it from birth or at around 4 years?
According to the Phantom Menace official novelisation, all children within the Republic are blood-tested at a very early age, typically just after birth.
But all Jedi were identified within the first six months of birth and given over to their training. It was true for him, for Obi-Wan, for everyone he knew or had heard about. There were no exceptions.
Like most Jedi, Obi-Wan is identified within six months of his birth and begins his training immediately so that he can learn to control emotions of fear and anger at an early age
The "The Jedi Path: A Manual for Students of the Force" factbook contains an extensive section on Jedi recruitment. Apparently blood-testing is mandatory and the Jedi have the legal authority to take a child into their custody if they're found to have significant Midi-chlorian counts. Apparently some children do slip through the net and are manually tested in the same way that Anakin was.
Locating Force-strong newborns is a straightforward process, at least within the Republic’s borders. Mandatory blood tests performed at birth record the concentration of midi-chlorians in an infants cells, and positive results are forwarded to the Jedi Temple for follow-up.
Because midi-chlorian analysis is not always definitive — particularly among older children or beings with unusual physiologies — special tools or puzzles may be employed instead. These include the testing screen, a tool that records a subject’s ability to read minds or view images remotely, and the mental maze, a test that demonstrates whether a subject can use rudimen¬tary telekinesis.