This Baby Blues strip was published back in early May (when the tulips really were in bloom), then MyActualRealName submitted it as a CIDU in early June, so it seems perfectly appropriate to post it here in early August:
M.A.R.N. commented: “Isn’t Hammie like six, and will have seen her like this about her tulips, and when toothpaste is on sale, many times?”
Perhaps, but I think the authors just didn’t have any other option, since the resident “baby” (Wren) is only about two years old, and thus far too young to deliver Hammie’s the reaction in the third panel. In addition, since the characters have been getting older (but only very slowly), it might be difficult to preserve exact consistency to their “current” ages. I don’t follow “Baby Blues”, so I can’t judge how well the continuity has been handled in the past.
Comics I Don't Understand 
“A foolish consistency is the hobgoblin of cartoonists with recurring characters” – not Ralph Waldo Emerson.
Ernie Bushmiller was well known for not letting the consistency of his characters (Nancy, Sluggo) get in the way of a good gag.
If you follow Baby Blues, Hammie is generally clueless, so even at six he might not have even noticed previous to this.
Also, she may have only planted the tulips recently.