Almost all Android mobile devices available today are susceptible to hacks that can execute malicious code when they are sent a malformed text message or the user is lured to a malicious website, a security researcher reported Monday.

http://arstechnica.com/security/2015/07/950-million-android-phones-can-be-hijacked-by-malicious-text-messages/

And the vast majority of them will never get patched because Androids have to be patched by your manufacturer or your provider who has no incentive to support old phones.