<?php
if ($stmt = $link->prepare("SELECT username FROM registrations WHERE phone = ? AND age = ? ")) {

    $stmt->bind_param("si", $phone, $age); <==
    // bind placeholder (?) by sequence ie. first placeholder would be string (s) and use $phone value
    $phone = $_SESSION[phone];
    $age = 18; <===
    if ($stmt->execute()) {
        $result = $stmt->get_result();
        if ($result->num_rows > 0) {
            while ($row = $result->fetch_assoc()) {


            }
        }
    }
}