problem with starting lineup
 

Coach marks players 1 through 5 as starters. About 4 minutes into the game, it's realized that players 2 through 6 actually started. (so 6 started instead of 1).
Is it too late to penalize?

See 10-1-2 Penalty and read the penalty for 2a.

Is The Speaking Captain Starting ...
 

If a team changes a designated starter (with exceptions) after the ten minute time limit, then a team technical foul is charged. If a player starts, and that player was not designated to be a starter, the infraction has to be discovered, and penalized, before the ball becomes live to start the game. Once the ball becomes live, it is too late to penalize this specific infraction, and no penalty can be assessed. As a reminder, the ball becomes live when: on a jump ball, the tossed ball leaves the referee’s hand; on a throw-in, it is at the disposal of the thrower; and on a free throw, it is at the disposal of the free thrower.

Also a case play exists that says before ball is tossed scorer notifies referee of wrong starter. Play says if coach "elects" to start player not listed as a starter he can and it is a T. So you give coach choice. Correct starter or T? Doubt coach ever chooses T.

Also, starter in nfhs can be replaced cause needs to change illegal equipment, apparel, injury, illness. Player comes out to start with wrong color undershirt. Send him off to change and new starter allowed. No penalty. That part surprises some folks. Thx

So....to clearly answer my question, it's too late to penalize?
Thanks

Too late.