A good time would be 29 seconds.
The average 100 yard time for a 7 year old boy depends on which stroke they are swimming and the length of the pool. A typical time for a boy younger then 10 swimming the 100 yard freestyle is 1 minute 10 seconds.
It really depends on the person and how long they have been swimming. I'd say anything less than 1:35 is acceptable.
if he is on a swim team than about 35 seconds and if he is not and just trying it about 40 to 45
35 to 40 secs or look at this and decide how good you want to be. http://www.ymcaswimminganddiving.org/2011SC/results/2012%20QT.pdf
Most people at 13 go between 18:00 - 25:00. It depends on how much training you have.
A fast time would be atleast a 25.