Getting an early start is one of the best things you can do. That way, you can make progress before living too much of your adult life while indulging strongly in sense pleasures in the lay world. Not doing that will allow them to become much more deeply rooted. If you don’t become a monk in your 20s, you will get very used to indulging in sex, alcohol, food, relaxation, fitting in at the workplace, etc., and those types of things will become habitual.
Furthermore, you won’t really understand the defilements unless you become a bhikkhu or live as an eight-precept layperson in a monastic-type setting for some time (at least a few months). That is because you don’t fully come face to face with them without doing so, and you cannot understand the level of sway that they actually hold over your mind unless you do. So, without facing them, you don’t fully understand them, and without fully understanding them, you have a diminishing chance of uprooting them.
Therefore, I recommend that even if you can’t become a monk, that you try to live at a monastery under the eight precepts for some time (at least 6 months or so) in your early 20s, and try to keep the rules perfectly for that time (and before), and practice meditation while there (particularly to reduce whatever your largest defilement is as stated in the Visuddhimagga by the Venerable Buddhaghosa, and to reflect on the sense bases, aggregates, elements etc., in order to develop wisdom, also as stated in the VsM).
The point I made above about understanding the defilements fully is an important one. A metaphor of gaining intelligence about your enemy in warfare can be used to explain this. In fact, the word Arahant actually means ‘foe destroyer’ (they are not destroyers of external enemies, but of internal ones, i.e., the defilements of mind). In his terrific 2016 book, former tank commander and Colonel, Douglass A. McGregor writes:
Surprise in warfare will persist, which is why a more accurate and timely picture of the opposing forces’ intentions and capabilities always confers a winning advantage. Field Marshal Arthur Wellesley, the First Duke of Wellington, said it best: “The whole art of war consists in getting at what is on the other side of the hill.” Wayne Gretzky put it differently: “A good hockey player plays where the puck is. A great hockey player plays where the puck is going to be.”
Margin of Victory: Five Battles that Changed the Face of Modern War by Douglass McGregor
The book, by the way, is an excellent one if you want a good history of a number of important modern battles, infused with a tremendous deal of strategy and tactical knowledge.
Remember: Become a monk in your early 20s, or at least spend a few months living as a white-robed layperson at a monastery — meditating, and developing insight. You won’t regret it. You will also easily be able to return to laylife if you feel that is a better choice for you, and will be able to develop a career, etc.
Renaldo