Button btn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    btn = (Button) findViewById(R.id.btn);

    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            PegarDatos();
        }
    });
}


private void  PegarDatos(){
    Calendar c = Calendar.getInstance();
    int dia = c.get(Calendar.DAY_OF_MONTH);
    final int mes = c.get(Calendar.MONTH);
    int año = c.get(Calendar.YEAR);

    DatePickerDialog dpd = new DatePickerDialog(MainActivity.this, AlertDialog.THEME_HOLO_DARK, new DatePickerDialog.OnDateSetListener() {
        @Override
        public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
            Intent i = new Intent(MainActivity.this, DosActivity.class);
            i.putExtra("DiaN",dayOfMonth);
            i.putExtra("MesN",month);
            i.putExtra("AñoN",year);

            btn.setText(dayOfMonth+"/"+(month+1)+"/"+year);
        }
    },año,mes,dia);
    c.add(Calendar.SECOND, 1);
    dpd.getDatePicker().setMaxDate(c.getTimeInMillis());
    dpd.getDatePicker().setMinDate(0);
    dpd.show();
}

TextView tvEdad;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_dos);

    Bundle BN = this.getIntent().getExtras();

    int d = BN.getInt("DiaN");
    int m = BN.getInt("MesN");
    int y = BN.getInt("AñoN");
    CalcularEdad(d,m,y);
}

private void CalcularEdad (int diaNac, int mesNac, int anoNac){
    Calendar c = Calendar.getInstance();
    int diaAct = c.get(Calendar.DAY_OF_MONTH);
    int mesAct = c.get(Calendar.MONTH);
    int anoAct = c.get(Calendar.YEAR);

    int edad = anoAct-anoNac;
    if ( mesNac>mesAct){
        edad--;
    }else if (mesAct==mesNac){
        if (diaNac>diaAct){
            edad--;
        }
    }

    tvEdad = (TextView) findViewById(R.id.tvEdad);
    tvEdad.setText("Edad: " + edad + " años");
}