I have class like:
public class Test {
private String Fname;
private String Lname;
private String Age;
// getters, setters, constructor, toSt
To simplify things you should probably make your age Integer or int instead of Sting, but since your question is about String age this answer will be based on String type.
Assuming that String age holds String representing value in integer range you could simply map it to IntStream and use its IntSummaryStatistics like
IntSummaryStatistics summaryStatistics = testList.stream()
.map(Test::getAge)
.mapToInt(Integer::parseInt)
.summaryStatistics();
int max = summaryStatistics.getMax();
int min = summaryStatistics.getMin();