"As we all know grass always looks greener but if you are in late 30's and don't have a partner, you know its time for you to rethink. "
Secured Future
Having an established carrier and good amount of saving will accelerate your relationship. You will have fewer worries and money and you can even think of starting family.
Matured Partner
The more you age the more you get from life. Settling down in early 30's will get you more matured life partner. By that time you both would have seen a lot in terms of relationship and other matters. So you can expect a more healthy and matured married life.
Better Choices
By that time you would have experienced a lot of companies and you can easily differentiate what is good for you and what not which will help you easily pick the one who can make you happy.
Self Realization
As you age, you will know a lot about yourself. What you are good at what not? You will get to know your positive traits and negative faults. Self realization will not only boost your career but also help you choose a good partner for you.
we should not settle down ... never
well said @indiantraveller