Step 1: Identify the concept being tested.
We must find the life stage in which vocational, or career, interest mainly develops.
Step 2: Recall the mental prerequisites for vocational interest.
A genuine career interest needs abstract thinking, identity formation, and future-oriented planning, abilities that mature only at a particular age.
Step 3: Rule out infancy and childhood.
In infancy and childhood, choices are based on imitation and play; no stable career preference exists yet, so options A and D are eliminated.
Step 4: Examine adolescence.
Per Erikson this is the Identity versus Role Confusion stage, and per Piaget abstract reasoning blossoms here, so teenagers begin seriously exploring real occupations.
Step 5: Rule out old age.
In old age the focus shifts to reflection and retirement, not the fresh formation of vocational interest, so option C is eliminated.
Step 6: Conclude.
The stage that uniquely combines identity formation with future planning is adolescence.
\[ \boxed{\text{B. Adolescence}} \]