It depends on the tree species but generally around 150-250 years or older.

>Hardwood forests of the eastern United States can develop old-growth characteristics in 150–500 years. In British Columbia, Canada, old growth is defined as 120 to 140 years of age in the interior of the province where fire is a frequent and natural occurrence. In British Columbia's coastal rainforests, old growth is defined as trees more than 250 years, with some trees reaching more than 1,000 years of age.[9] In Australia, eucalypt trees rarely exceed 350 years of age due to frequent fire disturbance.[10]

https://en.wikipedia.org/wiki/Old-growth_forest