Can the field data type created in ArcGIS Online be changed?

Note: 
It is possible to change an integer field type to another field type using the ArcGIS REST APIs. However it has its own limitations and issues so it is not a recommended workflow. Only field properties can be modified. Refer to ArcGIS Rest APIs: Update Definition (Feature Layer) for more information.

No, it is not possible to change the data type of a field after the field is created in ArcGIS Online. Data types can only be selected during the time of the field creation. Alternatively, create a new field with the preferred data type and populate the existing field value in the new field using an Arcade expression. The workaround below provides the steps to add a new field, select the data type, and populate the field values in ArcGIS Online.

1. Log in to ArcGIS Online and click Content > My Content.
2. Browse to and click the hosted feature layer to open the item details page.
3. Click the Data tab to show the table.
4. Add a new field and choose the desired data type for the field. Refer to ArcGIS Online: Add a field for further instructions.
5. Click the header of the new field and select Calculate.
6. In the Calculate Field: <field name> window, click Arcade. The Arcade Calculator: <field name> window opens.
7. Copy and paste the following Arcade expression into the Expression box. Replace <FIELD_NAME> with the desired field. Click Test.

$feature.<FIELD_NAME>

8. When the Results tab displays the result, click OK.

The images below show the values of the Age field populated in the newly created field (Age_New) with the desired data type selected.