To each his own. Some would benefit from early marriage others would find it more cumbersome. The person is more important the time of their arrival.
The Prophetic guidance is to marry early and deviating from prophetic guidance results in nothing but hardship and loss for you.
Yes there are extenuating circumstances that prevent this especially in a society like today were the halal has been made incredibly difficult, but as long as you always sought marriage early and circumstances prevented you, there is no blame upon you.
My issue is solely with female's who had absolutely no desire to heed this prophetic guidance in preference for chasing dunya and giving there best years to a cooperation.
You cannot deliberately deviate from Prophetic guidance and expect success in life as a Muslim.
Speaking for the age cohort that you are discussing late 20s and early 30s. Opportunities are not all gone and those that remain in the pool of eligibles are not lower tier options, seems somewhat exaggerated. You don't know each females options, her history, resilence or general interest in marriage, or procreation for that matter. Hence a one-size-fits-all approach does not work.
If you fit the extenuating circumstances in the sense you always desired to follow this, but it didn't materialise for legitimate reasons, then you have nothing to worry about.
The capacity to effectively articulate your emotions and mitigate misunderstandings is stronger as one gets older, level of maturity as well and a more crystallized idea of what one is looking for and willing to tolerate (or so I'd hope).
I posted a topic not long ago showing the highest divorce rates are in the 25-35 age group and the second highest is 35+, while the lowest divorce rates are <25 group.
This makes perfect sense because there is far less psychological, emotional and physical baggage those youngsters carry in comparison to those other groups.
When your young with no experience, your soul is pure and your willing to fight tooth and nail to keep the relationship/marriage afloat, as opposed to later in life were you loose that tolerance as a result of multiple bad experiences.
This of course comes as no surprise to me because everything the Prophet commanded (early marriage) is good for us, regardless which angle you look at it from.
I digress, the girl is 21. She should not feel insecure and frantic energy isn't a good look. One doesn't have to resign themselves to taking a back seat in their life affairs but careful consideration for life choices is vital. Especially as women have the most to lose in a coupling especially an early one.
The vast majority of single mothers who married early are better off then the vast majority of spinsters wallowing in debt and all sorts of other psychological/emotional issues.
There is a Somali maah maah that naag garoob ah is better then a 27+ spinster for marriage, because the garoob is a traditional wife and always wanted to be such.
While the spinster is being driven by her biological clock with no burning desire for either child-rearing or a husband which is a defect in as a result of the times we live in today.