Hello Neplako keta,
I received the same mail from social security office couple of months ago. Don't worry about it dude. I was also afraid while I got that letter but that is nothing.
I called them at that time and told them that "I do not work". They said that this is just a regular mail to notify people how a social seccurity could benifit them and it is sent couple of months before your birthday. They told me not to worry about that.
Just Chill man...
No worries...just ignore the letter...You might call if you want but I am pretty sure they will answer you the same thing.
Regards,
Grenade