Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
43. It is a positive commandment to marry a woman in order to be fruitful and multiply
As it states in Beresheis (Genesis) 1:28, "And G-d said to them be fruitful and multiply (have children)." A person's intention should be to continue the survival of the human species. A man when he turns 18 becomes obligated in being fruitful and multiply. If he has passed twenty years old and has not gotten married, he has transgressed and disobeyed this positive commandment. If he is involved in Torah and is afraid that because of needing to earn a livelihood it will take away from his learning, he is allowed to delay, but still should not go beyond the age of twenty-five.
This applies in all places and at all times.