I have a table called `PEOPLE` with the following data:

```
MYNAME    AGE     MYDATE
==========================
MARIO     20      2015/02/03
MARIA     10      2015/02/02
PEDRO     40      2015/02/01
JUAN      15      2015/01/03
PEPE      20      2015/01/02
JULIA     30      2015/01/01
JUANI     50      2014/02/03
MARTIN    10      2014/02/03
NASH      21      2014/02/03

```

Then I want to get the average of age grouping the people in groups of 3 ordering by `MYDATE` descending.
I mean, the result that I'm looking for would be something like:

```
23,3 
21,6 
27

```

Where `23,3` is the average of the age of:

```
MARIO   20      2015/02/03
MARIA   10      2015/02/02
PEDRO   40      2015/02/01

```

And `21,6` is the average of the age of:

```
JUAN    15      2015/01/03
PEPE    20      2015/01/02
JULIA   30      2015/01/01

```

And `27` is the average of the age of:

```
JUANI   50      2014/02/03
MARTIN  10      2014/02/03
NASH    21      2014/02/03

```

How could I handle this? I know how to use `GROUP BY` but only to group for a particular field of the table.