Is there any harm for younger people to take an aspirin once a day.?

Depends. Like all things in medicine the higher the risk a pt is, the more the reward with therapy. If a pt is younger than 40 yrs of age and very little risk factors the the benefit will be negligable and may cause harm over the long run ( ulcers etc).
No benefit, no need. Never take a medicine that doesn't offer you a benefit as there is always the potential for side effects and harm. Aspirin can cause stomach upset, bleeding, and increases the risk of hemorrhagic stroke slightly. The landmark physicians health study showed benefit in men >age 50, specifically not in men <50. For women, the age is 65 (nurses health initiative).