The only way I see a possibility would be with the use of AI, to understand the user's message and save this information correctly in your custom field.

Hi, ​@James97!

As ​@rogerioaraujo already said, AI is one way to achieve this. But if you won't spend money for this little thing, you could also set the Data Collection just to numbers and customize the answer if someone send additional text to it:

OR... you could use Google Sheets integration and the REGEXREPLACE formula to do the trick.

1. Save the age in a custom field using Data Collection:

   

    

2. Create a Spreadsheet:
   Type the header for CONTACT ID, AGE and for the last column (AGE NUM), insert this formula: =ARRAYFORMULA({"AGE NUM",IFERROR(REGEXREPLACE(B2:B,"\D",),B2:B)}) 
   • Add an Action to your Data Collection sending your Custom Field using Update Row.
     - Use the System Field Contact Id as Lookup Value

     

      

   • It will save the contact's answer to Google Sheets and the formula will remove any character that isn't a number from the AGE.
3. Retrieve the AGE NUM from the spreadsheet using Get Row By Value.
   • Again, use Contact Id as Lookup Value to retrieve the contact's age from the column AGE NUM.
   • You can save it to the same Custom Field you're using to store the collected age

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4. Now you can use it as you need...

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Hope this helps!