Missy sales

Hello, I'm currently completing form 80 for my parents who have applied for a contributory parent visa. When my father was in his late teens he was convicted of a criminal offence and spent some time in a young offenders institute. He's been led to believe that as this occurred over 40 years ago it's been removed from any official records and therefore does not need to declare it to anyone, but do I need to declare it on his form 80? If I don't declare it I'm worried it might get discovered as the young offenders institute is listed as his place of residence on my mum and dads marriage certificate. Advice please? Many thanks in advance