Hi Adrien,
Yes ths is possible.
1 way is to not let these leads into the engagment program in the 1st place. So in the campaign that adds your leads to the program you would just set your campaign up so that it only includes the leads that you want.
The other way is that you would just need to create a smart campaign to pause or remove the leads.The smart list should meet the defined critera for the lead being too young then in the flow you could do "change program stream caadende" to Paused.
Then you would have another trigger campaign created to listen out for when they should be unpaused.
https://community.marketo.com/MarketoArticle?id=kA050000000LAVfCAO
Cathal