Generally, world-wide, there are still many ongoing wars, for example the Syrian Civil war, which started in 2011.
If it's about world-wide conflicts: the last World War finished in 1945, or 70 years ago.
If its about the US: the last time US was in a declared war was 1945; so also 70 years ago. But in Practical terms, US is involved in wars now, for example in Pakistan.