The simple answer is ASAP (As Soon As Possible) if it could be possible correct answer would be - yesterday.

The fact of the matter is - the young seedling tree does not have CO2 sequestering ability as a mature tree, in fact not even close.
So we can assume that for that tree to start sucking significant amounts of CO2 it is necessary at least 10 years. Yet, again this also widely varies based on the growth rate, age, species of the particular tree, soil and weather conditions.

“The rate of loss has reached 26m hectares (64m acres) a year; a report has found, having grown rapidly in the past five years despite pledges made by governments in 2014 to reverse deforestation and restore trees.” ^[1]

If we take that one hectare of forest can have 1000 trees, including the devastating impact of wildfires, and growth of population we will face an estimated loss of 0.5 trillion fully grown trees in the current decade.

If we assume that over the growth period of 30 years, tropical trees can suck on average around 35kg of CO2 per tree per year, that would be equal to 35 tons of CO2 per year per hectare (~1000 trees).
The amount of CO2 a tree sequesters during first 10 years is on average 11 kg of CO2 per tree per year (or approximately 5kg for the first 5 years).
For temperate forests that number would be even lower - for 10 year period, on average it would be around 4 kg of CO2 per tree per year. And for 5 year period, that number would be about 2 kg of CO2 per tree per year.

Therefore, more we wait, as trees need time to grow, we will face a more significant danger and the probability of human race extinction.