Final answer:
Little Eugene can walk up the stairs by the age of two years, managing one step at a time while holding on, and by three years, he can do it more steadily, one foot per step without assistance.
Step-by-step explanation:
Little Eugene will be able to walk up the stairs by the age of two years.
At this stage, toddlers typically can move sideways and backward, and they can navigate stairs on both feet, one step at a time, while holding on to a rail or someone's hand.
However, by the age of three years, their gross motor skills have improved to the point where they can climb stairs one foot per step without assistance.