i have a custom field in lead and opportunity as AGE__C.and age__c field in lead is mapped to account.but now i need the same age__c field to be mapped to opportunity also...how it can be possible..how can i write a trigger for this scenario????

i have a custom field in lead and opportunity as AGE__C.and age__c field in lead is mapped to account.but now i need the same age__c field to be mapped to opportunity also...how it can be possible..how can i write a trigger for this scenario????