We start by finding the probability that none of the 5 babies will be born early, using the fact that the probability of any one baby being born on time or late is 0.5 + 0.3 = 0.8:
\begin{align*}
P(\text{none born early}) &= 0.8 \cdot 0.8 \cdot 0.8 \cdot 0.8 \cdot 0.8 \\
&= 0.32768.
\end{align*}
Then, we can find the probability that at least one baby will be born early by subtracting this probability from 1:
\begin{align*}
P(\text{at least one born early}) &= 1 - P(\text{none born early}) \\
&= 1 - 0.32768 \\
&= 0.67232.
\end{align*}
Therefore, the experimental probability that of 5 babies born, at least 1 will be born early is about $\boxed{\textbf{(D)}\ \frac{4}{5}}$.