My hair has a variety of textures and curl patterns, even. My underlayer is thin and stick straight. The rest of my hair is mostly medium texture. Maybe about 5% of my hair is thick, coarse, and somehow kinky. I wouldn't think you'd have anything to be worried about as long as you're not experiencing any excessive hair loss.

Here's an interesting article:
Why Does Hair Texture Change Throughout Life?

My ponytail has gotten smaller too over over the past 10 years (I'm 24). I haven't experienced any noticeable hair loss though, so who knows? It might just be getting older.

My hair is always flatter in the morning after sleeping on it. I sometimes braid my hair or put it in a low ponytail during the day in an attempt to make it less poofy/voluminous when I let it down. So yes, I guess all of that is normal for me. Have you tried pineapple-ing at night?
Wave Pattern: 2A
Hair Properties: Normal/Low Porosity, Medium Density, Medium Texture w/Fine Straight Underlayer
Co-wash: Suave Naturals, Cure Care
Low-poo: Renpure Brazilian Keratin
Rinse Out & Leave In: Tresemme Naturals
Gel: LA Looks Mega Mega Hold Gel, Garnier Fructis Spray Gel