After 1907, but probably before the first World War.
Pictorial Review offered this pattern in sizes from 4 through 16 years. This one is a size 8 years. Boys older than 16 would be able to wear smaller men's sizes.
These overalls are the right thing to wear when helping to put in the vegetable garden. It would seem that transplanting flowers, as this young gentleman appears to be doing, is perhaps a more formal activity, requiring a beautiful spotted blouse and a tie.
Pictorial Review patterns were advertised through the Pictorial Review magazine, and like all fashion magazines, the publishers advertised the fashionableness of their offerings. But clearly somebody felt that a highly functional overalls pattern would find an audience.
A separate layout sheet is included. The patent date of 1907 is for the layout, not the style.
But the instructions are still given on the front of the envelope.
This pattern was closed with a pin, something I find now and again, most commonly with patterns before the 1920s.
Although the back of the envelope is unprinted, the back of the layout sheet offers some advertising.