Sparkly, I understand why that was offensive to you, but at the same time, I think you should understand that Shopping613 had no way of knowing that you would be offended by her comment (since she was sure it was the truth). If you did in fact feel offended, it makes sense to let her know politely since she has no way of knowing o/w.
btw, people always think that I am way younger than I am (since I both look and sound young). Even though it is a compliment (because it is based on the fact that I look young), it is hard not to be slightly insulted sometimes. But I understand that they are basing it on my appearance and my voice and my style of speech and that it has absolutely no bearing on my intelligence or maturity.
In this case as well, I think that Shopping613 was referring to your style of speech, and that does not reflect on your intelligence or maturity. In fact, some people consider it a maaleh to have a youthful manner of speaking.
But again, the main point is that Shopping613 did not mean to offend you and had no way of knowing that it would be offensive.