>>A good age difference is..between 12 to 18 years

But majority population of your homeland would not consider this age gap as an appropriate. From what I see from the statistic, people in Australia choose spouses 1-3 https://dmmng.com/manage_social/centeryears older/younger. And 15-18 (!!) years age gap is huge, I would not consider it either.

Because "[in Australia] The median age of males and females at marriage was 31.9 and 29.9 years respectively in 2016"