In your forties
Definition 1 of 1
Pro Tip 1/3Age Implications
The phrase implies an individual is not young but still not old, often seen as middle-aged. 
Even in your forties, you can start a new career.
Pro Tip 2/3Cultural Perspectives
The perception of being in your forties can vary widely between different cultures and societies. 
In some cultures, being in your forties means you're expected to have life figured out.
Pro Tip 3/3Casual Conversations
Used in informal settings to discuss age without specifying the exact number, often to avoid precision. 
I'm planning a trip with friends who are all in their forties.