One Hot Encoding of Age
The task of predicting how many years a person has left to live is called survival analysis. Survival analysis is a type of time to event analysis. Thus, survival analysis needs a special type of loss function. An appropriate loss function would avoid predictions like 50 years left when the current age is 70. A common survival analysis model is Cox regression model which can be adapted to neural networks.

If survival analysis is used, the current age can be inputted in the model directly in a single input node.