"in tech" is a very large spectrum of industries, positions and earning power.
A programmer at the age of 30 is probably not on a decline, but will definitely not see any major pay bump in the future;
A consultant is probably just at the beginning of a consulting career;
A mid-level manager has probably just established a foothold in the corporate pyramid, and there is a long way up;
A founder is probably hoping for (more)success in the future;
The CEO of Facebook may be at his peak now, but he probably doesn't care.
A programmer at the age of 30 is probably not on a decline, but will definitely not see any major pay bump in the future; A consultant is probably just at the beginning of a consulting career; A mid-level manager has probably just established a foothold in the corporate pyramid, and there is a long way up; A founder is probably hoping for (more)success in the future; The CEO of Facebook may be at his peak now, but he probably doesn't care.