The total number of participants is 750.
- 450 are females.
- Thus, the number of males is \( 750 - 450 = 300 \).
- 1/4 of the male participants are less than thirty years old, so the number of males less than thirty is \( \frac{1}{4} \times 300 = 75 \).
- Half of the females are less than thirty years old, so the number of females less than thirty is \( \frac{1}{2} \times 450 = 225 \).
Thus, the total number of participants under thirty is \( 75 + 225 = 300 \).
The probability is the ratio of participants under thirty to the total participants:
\[
P(\text{under 30}) = \frac{300}{750} = \frac{1}{3}
\]
Final Answer:
\[
\boxed{\frac{1}{3}}
\]