**Average age is 12 years and 3 months**

1 year is having 365 days (assuming only in leap year)

1 day is having 24 hour

1 hour is 60 min

1 min is 60 second

**Now doing all these in reverse order,**

1 hour = 60 x 60 second

1 day = 24 × 60 × 60 second

1 year = 365 × 24 × 60 × 60 second

So, 12 year = 12 × 365 × 24 × 60 × 60 second = 378432000 sec

1 months = 30 days

3 months will be 3 × 30 days

= 3 × 30 × 24 × 60 × 60 second = 7776000 sec

12 years and 3 months = 378432000 + 7776000

**= 386208000 second**