What does this "one year out" mean?
What did the four young men of Take That think?
They had been all set to go as a quartet when they suddenly discovered
they were about to have a new boy in their midst.
"This picture looked like 14 years old school kid" said Gary
He was only one year out.
* a new boy=Robbie Willams Thanks in advice

Question

What does  this "one year out" mean?
What did the four young men of Take That think?
They had been all set to go as a quartet when they suddenly discovered
they were about to have a new boy in their midst.
"This picture looked like 14 years old school kid" said Gary
He was only one year out.
* a new boy=Robbie Willams      Thanks in advice

Answer

Apparently, there was an age requirement for being in the group. This new boy was only one year from being old enough. ("one year out") It could also have meant that the boy was only one year away from finishing school.

Answer

"This picture looks like a 14 year old school kid!" said Gary
He was only one year out.

"one year out" in this context means that Gary's guess was one away from being correct.
Robbie was 15 years old and Gary had guessed 14 years, so we say that; "He was only one year out.".
If Robbie would have been 17 and Gary had guessed 14, then Gary would have been three years out.