I haven't established an exact age for Maggie. But she's definatly an adult as stated by Lisa's obversations in the Christmas Special Part 3 https://youtu.be/x4HhvvMVqyU?t=1m23s So she was definatly born before 2038.
Also in the 2009 Simpsons comic, a different version of Maggie is seen. With a dead baby maggie wrapped up in newspaper on the floor. This implys that the adult maggie killed the original and took her place. I never stated this in the animated version but something similar did take place.
I noticed the Maggie carcass in the Snospis Eht comic 2009, but I always thought that Maggie had just transformed in some way where she did not need her skeleton anymore. However, it may have been mentioned in the Snospis Eht: Christmas Special: Part 03, as Lisa said "A few days ago, Maggie seemed smaller and less human. Now she looks more like a fully grown adult male", implying that Maggie has gone under some form of transformation.
I just noticed you actually said "Marge's age" in your question. My apologies, I misread it as Maggie.
I always saw Marge as being around 40 during the events of the first xmas special in 2056. So she would have been born sometime around 2016. In fact lets just say she was born today. 21st september 2016, this is now canon, im declaring it so.