How to get count of number of rows where one of the columns entry is the same

You can use a GroupBy statement for this to group by a value of your items, and then Select the result you want from it:

var result = await db.Model
        .GroupBy(x => x.Age)
        .Select(g => new {
            Age = g.Key, 
            Count = g.Count(),
        })
        .ToListAsync();

The result is a list of objects that have an Age property with the age value, and a Count property with the number of items that had that Age value.

If you just want the counts, then you can just return those from the Select expression directly:

var result = await db.Model
        .GroupBy(x => x.Age)
        .Select(g => g.Count())
        .ToListAsync();

Note that this will obviously prevent you from saying what age an individual count is representing.