Technically, Lal Mirch would be 12 years old since she was 2 as an arcobaleno in the present. But the arcobaleno probably age differently than humans, so no one really knows for sure. Great question though.
It depends on whether you mean physically or realistically. It has been revealled that Arcobaleno don't age so although Reborn has said that he is 2 years old, he even had a birthday party, this is definently a lie.
Working a little roughly: The first Sky Arcobaleno, Luce, has been shown to have been pregnant with Aria before becoming cursed. Assumming Aria was between 20-30 years old when she gave birth to Yuni and that Yuni was around the age of 15 in the Future Arcs that means that atleast 35 have passed since the Arcobaleno were cursed, and possibly over 45 years. Add to that how old you think that Lal (or any of the Arcoabeno) were at the time they were cursed and you will have there 'mental' age during the Future Arc. So, if you believe that Lal was around the age of 20 then she would mentally be between 55 and 75 years old, depending on how old you believe Aria is during the Future Arc. Also, obviously take away 10 years and you will have their current 'mental' ages.
Physically speaking, I would theorticize, that when the other Arcobaleno were killed that Lal's curse was essentially lifted and that she returned that the form that she was in before she was cursed and she started to age again. Therefore, if you believe Lal was 20 when she was cursed she would be 20, plus the difference between when the curse was lifted and the Future Arc started, physically in the Future Arc.
The only problem I can see with the math of my 'mental' theory is the new information that was come from the Vindice, who have said in chapter 386 that new Arcobaleno are created by (at least) two alternating methods far enough apart that no one can remember them, however 100 years, even 160 years isn't actually that far apart. Especially not for the Mafia, a group that is set in traditions and remember feuds for generations. However, it is still possible the information from the Vindice in incorrect or incomplete.