Ukrainian President Volodymyr Zelenskiy signed a bill lowering the military mobilization age by two years from 27 to 25.
Kiev faces heavy losses on the battlefield after two years of war, while Russia enjoys a considerable advantage in human resources.
The move will allow Ukraine to call up more people to replenish its reserves, as the number of volunteers has declined.
The bill was passed by a vote of parliamentarians in May 2023, but did not go into effect because Zelenskiy did not sign it.
It was not immediately clear what prompted the president to sign the bill on Tuesday, but he had previously warned of plans by Russia to potentially launch an offensive this spring or summer.
Zelenskiy's signing of the bill could mean Ukraine's efforts to build strong defensive fortifications in case of a Russian attack.
The Ukrainian military hoped not only to cut off Russian supply lines to Crimea, but also to regain some of the territory Russia had gained.
But lacking air superiority and facing a formidable Russian defense, Kiev's counterattack ground to a halt in early winter, with fears that Russian forces could overwhelm Ukraine.
Last year, Ukrainian General Oleksandr Tarnavsky warned that the military had already been forced to scale back some military operations due to reduced foreign aid.
Ukraine is highly dependent on Western supplies, especially long-range missiles and air defense systems. But billions of pounds of US and EU aid have been put on hold amid political rows.
Russian President Vladimir Putin said more than 600,000 of his troops were fighting in Ukraine, but he did not say how many had died there.
A confidential U.S. intelligence report released in December estimates that 315,000 Russian soldiers have been killed or wounded since the war began, nearly 90% of Russia's military personnel at the start of the invasion. There is.
Zelenskiy said in February that 31,000 Ukrainian soldiers have been killed since the war began, but U.S. officials say at least 70,000 Ukrainian soldiers have been killed and up to 120,000 wounded.