(I don't know the Peg o'My Heart reference, but I'm assuming you're saying she's too young for the role?)
The Macbeths have been played by actors of many ages, depending how the director interprets the role. Are they young and hubristic, easily manipulated by temptation? Or are they older, jaded, and frustrated with their station in life after decades of being in the same place? It really could go either way. Ronan is 27 now - a bit younger than your average Lady M, but not prohibitively so, especially if it's incorporated into the direction like I said. I think it would be weird to see someone in their early 20s do it, but late 20s feels grown up enough to pull it off.