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 60min

1 min is 60 second

Now doing all these in reverse order,

1 hour = 60×60 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