I'd tell them to go for it, but make sure you understand what you're signing up for.
This trade can completely change your life. It can give you opportunities, financial freedom, experiences, and a sense of accomplishment that's hard to find anywhere else. There's something pretty incredible about looking back at a job and knowing, "We built that."
But don't get into it just because you heard linemen make good money. The money comes with sacrifice. There are going to be long days, nights without sleep, time away from home, layoffs, traveling, bad weather, and plenty of moments where you're going to question why you chose this career.
Come in humble. Work hard. Ask questions. Listen to the older hands. Don't pretend to know something you don't, especially when your safety and the safety of the people around you depend on it.
And if you stick with it, enjoy the process. I spent years trying to become a lineman, and sometimes you get so focused on reaching that next step that you forget you're already living the experience you once wanted so badly.