'I want 12 children to come up on stage.... Right, now these 12 will be broken down by age and sex - well, it happens to us all in the end.'
The clause beginning 'Right' uses the verb in the sense 'analyse or separate'. The punch-line interprets it in the sense 'make to deteriorate through wear and tear'.
PS Sorry, it's not separated there . If it were separated ('I will break them down by age and sex') the joke wouldn't work because it depends on the phrase 'broken down''.