Every1 is different. It is quite safe to use appropriate Vaginal Atrophy every night if required, we cannot over dose on oestrogen. It is a natural hormone which because it drops, the body may become dry inside and out! Scalp, skin, between the toes, deep in the ears, nostrils, eyes, vagina, anus .......... I don't think it's time related as to when we up/drop the HRT regime, too many forget that H-'replacement'-T.
Sometimes it takes a while for a very dry vagina for the walls to absorb treatment so perseverance is important. Any stinging should be relieved too.
Some find that a moisturiser i.e. Sylc or Yes might help on the outer labial area - it can be a bit Trial and Error until we find a regime to stick with.
No-one is too young

2 be in perimenopause. Some girls never menstruate, others have a few periods then nowt.
Hopefully this GP will be educated by contacting the menopause clinic which mayB quicker than asking for an appt. yourself. There is that option though if she isn't supportive.
The dry / itching will change if treatment is given and used when required. The initial idea is to use every night for 2-3 weeks then as necessary: some require continual nightly insertion/application, some manage symptoms with every 3rd and 5/6th night. Occasionally as oestrogen levels continue to to drop we need to re-load to every night to get over the feelings. 4 me it was the need2P continually when I would sit on the loo with a cuppa in one hand and glass of cold water in the other to allow the muscles to relax.