My PD said you can start as early as 1 week. :) To make the math cards you need to get 250 gsm paper and cut it into 11x11 inch and you can buy the red dots from popular. The dots should be 19mm. They sell it in packs of 100. After that laminate the cards. It took me about 1 week to do the 100 math flashcards - from no 1 - 100.
The glen doman website has classes starting from 3 months - so I am guessing 3 months? Most of my friends started at 6 months though. I plan to start from month 2.
For Math, the best time to start is 3 months. They really need to be young to be able to learn to "see" quantity.