I’m an 18 year old female and I wanted to expand my range (both higher and lower notes) and I was wondering how I could do so without putting a strain on my vocal cords. I always wanted to know, is it too late at my age to sing in the whistle register? Just three years ago I was able to sing in the whistle register but I’m unable to access that part of my voice now.
You can definitely sing in your whistle register. However, your voice has just changed due to puberty and so you might have to find it anew. I recommend you work with a teacher. Don’t worry too much about expanding your range — focus instead on having a lot of breath support, clear, easy vocal cord function and complete resonance. Once you develop these elements, the range will naturally increase.