ANSWER:

And who has given them the guarantee that they would live to become old or live to ask for forgiveness! Allah has admitted a woman to hell simply because she kept a kitten locked up till it died! Allah ordered the hand of a thief to be amputated for stealing what is less than 10 dollars. Allah ordered the adulterer to be stoned to death for inserting few inches in what is not permissible for him! Do they think that after doing all these heinous sins, depending on Allah’s mercy that Allah will let them go?

These are all whispers of Satan to make them fall so deep into sin that they would not be able to get back up from there.
Nobody knows when their time is written, one must repent now and ask Allah for forgiveness and live as if it’s our last day of life. 

What advise can you give to someone, who say that, it is ok for them to enjoy the music, the social life, alcohol in theirs early age, then do the repentence later on in life, rather than being steadfast in deen early on, due to reason that Allah swt will forgive all our sin if we ask for it no mater where we are in life.