Why were the ABC and ENIAC considered first generation computers?

Answer 1

They are first generation computers because they used vacuum tubes as active elements, as did almost all computers until 1958 when the first generation is usually considered to have ended. From 1958 on most computers used discrete transistors as active elements until 1964.

Note: ABC & ENIAC are both very early first generation computers as they used ordinary Radio receiver vacuum tubes, not the more expensive Computervacuum tubes that were introduced about 1948 to 1949. The later Computer vacuum tubes were custom redesigned versions and were far more reliable than ordinary Radio vacuum tubes had been.

Answer 2

Both used vacuum tubes as their active elements, as did other computers until about 1958.