At what age do you consider a person to be "almost 30"? As late as 29 or as early as  25 /24? I'm curious cause I've heard varying answers before

Any time after age 29 but that's only an opinion.

Of course, if I apply that to myself I'd have to confess *shudder* to being almost 80.

At age 29.

Never really thought about it before.

A quick "review" of the internet seems to favor the 29th year of one's life.

The answers you have received so far bear that out as a common assumption.