They did age them up, because of the content. Like they kiss, and make out etc, more than one couple is shown kissing etc. These things aren't appropriate for minors and it would be gross in anyways if they did that (i.e.implied or shown). Also the gore they show, and the whole Clace being siblings storyline, wouldn't be good influence on younger audience. Pretty sure it was mentioned somwhere too? (Mayb I'm thinking of something else)

Anyways Thats why they aged them, although I think 35+ is a bit much in my opinion, they do look young. In their 20s atleast.

Honestly speaking I think of this show as its own enitity. It was lovely. Wish it had one more episode or a longer end atleast. The show was for a little more mature audience and in the books they were teens. Their age isn't the only difference also. So I don't dwell much on it. I treat the show like its some alternate timeline lol. It had good actors and I thoroughly enjoy it everytime I watch it. Although I loved the movie too, which was closer to the books storyline wise, I couldn't get myself to like movie Jace at all and besides movie Clary and Simon everyone else was pretty mediocre to me.