user.js

router.get('/:page', (req, res) => {
  let limit = 50;   // number of records per page
  let offset = 0;
  db.user.findAndCountAll()
  .then((data) => {
    let page = req.params.page;      // page number
    let pages = Math.ceil(data.count / limit);
		offset = limit * (page - 1);
    db.user.findAll({
      attributes: ['id', 'first_name', 'last_name', 'date_of_birth'],
      limit: limit,
      offset: offset,
      $sort: { id: 1 }
    })
    .then((users) => {
      res.status(200).json({'result': users, 'count': data.count, 'pages': pages});
    });
  })
  .catch(function (error) {
		res.status(500).send('Internal Server Error');
	});
});

Maybe it's req.params.page instead, no? https://gist.github.com/amandeepmittal/e49845cb40a6e1b1ff9518209f25bf95#file-user-js-L6

Yes, indeed. My mistake.
Correction done, thank you!