Regarding the Definition,
"adult" means a person who has completed his fifteenth
year of age;
It means The Act considers the individual who has completed his 15 years of age, Please do not confuse it with the MAJORITY (18years).
The Act concentrates only on the required details as required under the situation which fall under the provisions of the Act, they not have there general application but for the purpose.
The Constitution, Indian Penal Code and both Civil And Criminal Procedure Codes (unless exempted under those laws) have general application through out India. But not those statures which are enacted for the specific purpose, The Factories Act to regulate the law for given situations. Law is a tree where as an Act is a branch, tree is treated as a whole applicable to all, but the branch only to the extent of that specific branch. So where ever you find difference under such Acts or rules, you shall give its meaning in specific to that particular purpose only.......
Hope i made it clear to you with more than necessary explanation ;-)
Have a nice day.........