Dreaming about someone from your past, especially in a romantic context, can symbolize unresolved feelings or nostalgia for that time in your life. Since you mentioned that your dreams feature this girl as a grown-up, it might indicate a longing for innocence or simpler times in your life.
The fact that you don't consciously think about her in your day-to-day life suggests that these dreams may not necessarily be about her specifically, but rather what she represents—perhaps the feelings of admiration, love, or even childhood innocence and connection.
It's not uncommon for dreams to bring up past relationships or feelings, even if they don’t actively affect your current life. This could be a reflection of your subconscious exploring these emotions rather than a direct indication of something unresolved.
If it doesn’t bother you in your waking life, it might not be necessary to deal with it directly. However, if you find that these dreams are frequent and evoke strong emotions, it might be worth reflecting on what they mean to you personally. Consider journaling about your feelings or the dreams themselves; this can provide clarity and insight into why your subconscious is revisiting this chapter of your life.