How old should you be when you begin Jedi training?

According to the (fully canon) Ultimate Star Wars factbook, your training should begin almost from birth.

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

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The excellent (but alas, no longer fully canon) Star Wars: The Jedi Path: A Manual for Students of the Force also offers us some pretty solid insight about the early life of a Jedi initiate.

Younglings who have grown up within the Temple hardly need guidance on what to wear. You've been wearing a uniform since you could stand.

It seems that the normal age for a new arrival seems to be well before the age of 2-3 years. Some (like Ahsoka) can end up arriving as late as 8 or 9, presumably depending on the level of Jedi presence on their world but this is unusual and undesirable:

The responsible use of power takes a lifetime to perfect, and therefore the Jedi Order only rarely accepts members who are older than a few years. Most of you came to the Temple as infants.

My opinion, based on the other Star Wars stories (not the last three movies) is about 5 to 9 years old was the age they were inducting them into the Jedi Academy.

I think it was Mace Windu who was actually inducted shortly after birth but that was abnormal.

You are missing entirely what kind of training this is. It was one using physical bodies but the essence of the training was spiritual (of the spiritual force).

Anakin had an older "more experienced" spirit than anyone Yoda had previously trained, a spirit that was much more aware of the spiritual world's workings than any of Yoda's disciples, and perhaps Yoda himself.

It was not that Yoda did not trust Anakin's spiritual ability; Yoda realized that neither he, nor his own trainees had little to offer Anakin.

Anakin even realized, once Obi-Wan started "training" him, that he felt that he was being held back.

What that has to do with anyone who is more spiritually aware "mature" than their Pastor or mentor, they are shortchanging their own Holy Spirit inside them to trust in those who are spiritually weaker than them to lead them.

I hope I have hit the "too old to train" issue on the nailhead.

The Jedi wanted to get their younglings in infancy before the normal familial attachments were too strong, in their opinion. When Anakin was interviewed by the Jedi Council, it was a special consideration as otherwise he'd had been rejected out of hand as a matter of policy. I have jokingly envisioned a scenario where a Force-sensitive mother is birthing her child. Yoda wants with the midwife right there to take the baby. As the child enters the world, the midwife drops it! Fortunately, the baby is alright, but Yoda turns away, muttering, "too old, the child is...too old for the training!"

Although I can’t claim to know the details of the Star Wars universe, the dialog excerps that you have quoted do not strike me as particularly contradictory.

You are assuming that there is a specific age at which all children would be considered “too old”, but this may not necessarily be the case. It could be a notion that is individual to each child. It could well be the case that whenever they say Anakin is “too old”, they mean that he could have been trained earlier but now it is too late for him; but whatever it is that made Anakin “too old” at age 9 may not have applied to Luke at the same age.

The fact that there are several references to “too much fear/anger” etc. seems to corroborate this notion. Anakin would be “too old” (= too habituated) to unlearn these things, but Luke might not.

In Star Wars Episode V: The Empire Strikes Back, Yoda said, "You must unlearn what you have learned".
Plus, with our increasing age, we learn automatically from environment (like we can't lift stones in air from a distance).
So, the age should be around 2 or 3 when mind is fresh & empty to learn mysterious things. For each kid, its different as some start learning quickly & some slowly. So, even 2 or 3 years old kid needs to be judged by Jedi council.