Hi navid,
First of all, congratulations!
As for your question, you already had answers from other postings. I understand that you are worried, but people don't like multiple postings on the same topic...

About age out, it's going to be a problem when children turns 21 way early before inteview.
In your case, you don't need to worry about anything if your interview would be scheduled in Oct or Nov 2007,
your son (well, I think it's actually YOU) is still 20 at the time of inteview, right?
For those who are worried about the same issue, read my answer below, and just calculate like example.
http://www.immigrationportal.com/showpost.php?p=1664257&postcount=49
You just need to find out "pending duration" between October 4 (1st date for DV-2008 entry) and date on NL. If child has been 21 less than the pending duration before interview date, then s/he can benefit from CSPA. In other words, visa will be given to the child, too.