How old is my Guild AD406699 - Guild Gad25 Dreadnought Acoustic Guitar

Check out https://guildguitars.com/g/starfire-v/

The 6 digit number system was used from 1974 to 1979. Your guitar is a 1979. Depending on condition, it would sell for about US$2000.
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Normally it's on the back of the headstock on vintage Guild guitars

1979

Your guitar was made in 1973. Without more information as to the model and condition, i can't offer an opinion on the value.

Your guitar was made in 1960

I can't find that one in the Guild website.

Can you tell me the model number of the instrument?

Because according to the Guild database, it doesn't exist.

Is it possibly one of those guitars "by Guild" like the Madeira, or the DeArmond?

GAD D-25 guitars were initially made in 2007

If you're the original owner (and have the reciept), get ahold of a guild authorized store near you- they'll either repair it, or send it to be worked on at the factory- and all you pay is shipping.

If you're not the original owner, I would *strongly* suggest having it done by a professional.

Try this link:http://www.guildguitars.com/support/contact_us.php