I once read a dermatologists paper that said all men lose density even slightly from between 20 - 30, including the men that will keep their full head of hair for the rest of their lives and this is simply a form of the aging process. At 25 years old you will be bang in the middle of that range so thats what it could be.
You have a good hair coverage and low forehead so all looks good really from these pics- you could try adding Minoxidil to the areas you are concerned about and include taking a supplement like Biotin and together these could help maintain the thickness? Worth a try!