I don't know if it's ever possible. I always felt like I was about 14 round my mother. Sometimes she treated me as if I was a lot younger - we were at a family thing when I was in my 20s, and I was asking for sponsorship for a charity thing I was doing. A fairly distant cousin gave me a £20 note, and before I had time to draw breath, my mother was saying, "Say thank you!" If there had actually been a long pause, perhaps it would have been fair enough, but there really hadn't been. I suppose at least she didn't say, "to the nice man," afterwards. Indeed, she once said she was talking to someone at work, and realised I was older and I did a more responsible job than the person she was talking to, but she still couldn't really think of me as more than about 16, whatever she really knew.
In some ways, that's a good thing, I suppose - your mum's always your mum. Except that thinking of me as a teenager meant she was always thinking of me as difficult and argumentative, (I wasn't particularly bad as teenagers go, but there was a lot of stuff projected on me for various reasons,) and not someone she was prepared to listen to, and it took quite a bit of strength not to fall back into that pattern, and I didn't always succeed.
Also, when I really was 16, that was the year I ended up with 6 stitches after she hit me, so while in some relationships, thinking of your daughter as she was at 16 might not be so bad, I really don't think it was helpful in our particular relationship. It's one thing I don't miss now she's no longer here.