No one knows. Google doesn't really provide any useful information on the matter - they simply pretend to. Then all the SEO folks pretend to understand the issue. In short: don't worry about it. Just let things take their (un)natural course.
Please remember there is a no site specific policy in this forum. Questions related to your specific website can be posted in the Site Reviews forum.
Also ... Google has never acknowledged or not acknowledged that a sandbox exists. However, it has been confirmed that domains that have existed for many years often rank higher, most likely because spammy, poor content domains come and go so easily nowadays, and the best sites are the ones that stand the test of time.