Tropical rainforests are very important from many different points of view, first of all they produce somewhere between 20-30 % of the world's oxygen, they absorb carbon and help regulate climate, and they also contain 80 percent of the planet's terrestrial biodiversity. Long long time ago forests use to cover 20 million square kilometres, today 12 million were already cleared, and another five million have been selectively logged.
Latest satellite observations have shown that about 350,000 square kilometres of the original forested areas are growing back which is very small compared to size of cleared areas. The good news is that this number could increase in years to come because of large-scale migration of people from rural areas into cities in tropical areas. This would give new forests more areas to grow and could provide habitats for rich biodiversity that would otherwise be lost permanently.
However in order for second-growth forests to provide shelter and habitats for different animal and plant species they have to be connected with old-growth forests so species can move to them. Of course old ancient forests are of the best quality, for instance they are not so fire prone like secondary forests because they are more humid.
Even though second-growth forests have potential to at least partially replace old growth forests it has to be said that main causes of deforestation such as logging, mining, industrial agriculture will also harm secondary forests. But of course the most important thing of all is to preserve old ancient rainforests because deforestation of irreplaceable old growth forests is happening faster than any time in history (Borneo, Indonesia, Amazon rainforest).