As we all know that a subdomain is like a new website. So whether google will sandbox just the subdomain only or entire website along with all other subdomains and pages. For example website mydomain.com is 1 year old website sub1.mydomain.com is 8 month old and the third one sub2.mydomain.com is 15 days so in that case what google will do. will it sandbox just sub2.mydomain.com or all the three website. ?
It depends on how much overall-authority your domain has, usually your sub domains will get sandboxed. But I would never recommend you try to rank for different keywords with sub domains, you mess up 1 time, and everything will go down. be careful...