Levine School of Music.

Most of the programs I've talked to (we're looking for guitar lessons for a 6-year-old) start at 6 or 7. School of Rock has a "Rookies" program that I think starts at 6. You might also see if there is a Music & Arts branch, or if you can find a class through Montgomery County - I know Arlington and Fairfax offer group classes.

LMN Music comes to your home. http://learnnowmusic.com/
You can check with Academy of Fine Arts and Music in Gaithersburg to see if they still offer private guitar.
Bach to Rock at Washingtonian/Rio
Victor Litz in Gaithersburg

House of Musical Traditions in Takoma Park.

Thanks everyone!! Good to know best age. I had no idea.

If you have a girl 5 might be ok for starting guitar - probably better ukulele though. For boys its not recommended before age 7 because of nerve development in their fingers, plus strength etc.

Not sure if he'll come to you, but we did a music class with Steve through Miss Amy's Music Makers and he was wonderful and also gives private lessons. I'd look into it.

Check out Bach to Rock for guitar lessons. They have a location in Bethesda https://bethesda.b2rmusic.com/ and Gaithersburg https://gaithersburg.b2rmusic.com/.

There hands are not strong enough to do the fretting, IMHO

In MOCO. Thanks for suggestions!